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((short description|Process of preparing a data storage device for initial use))
'''Disk formatting''' is the process of preparing a [[data storage device]] such as a [[hard disk drive]], [[solid-state drive]], [[floppy disk]], [[memory card]] or [[USB flash drive]] for initial use. In some cases, the formatting operation may also create one or more new [[file system]]s. The first part of the formatting process that performs basic medium preparation is often referred to as "low-level formatting".<ref name="Tanenbaum">((cite book |title=Modern Operating Systems |edition=2nd |author-last=Tanenbaum |author-first=Andrew |author-link=Andrew S. Tanenbaum |date=2001 |publisher=Prentice Hall |at=section 3.4.2, Disk Formatting |isbn=0130313580 |url=https://archive.org/details/modernoperatings00tane |url-access=registration ))</ref> [[Disk partitioning|Partitioning]] is the common term for the second part of the process, dividing the device into several sub-devices and, in some cases, writing information to the device allowing an [[operating system]] to be booted from it.<ref name="Tanenbaum" /><ref>((cite web|url=https://docs.microsoft.com/en-us/windows/win32/fileio/disk-devices-and-partitions|title=Disk Devices and Partitions|date=7 January 2021|website=[[Microsoft Docs]]))</ref>  The third part of the process, usually termed "high-level formatting" most often refers to the process of generating a new file system.<ref name="Tanenbaum"/> In some operating systems all or parts of these three processes can be combined or repeated at different levels((refn|E.g., formatting a volume, formatting a [[Virtual Storage Access Method]] [[Virtual Storage Access Method#Linear VSAM organization|Linear Data Set (LDS)]] on the volume to contain a [[zFS (z/OS file system)|zFS]] and formatting the zFS in [[UNIX System Services]].|group="lower-alpha")) and the term "format" is understood to mean an operation in which a new disk medium is fully prepared to store [[Computer file|files]]. Some formatting utilities allow distinguishing between a quick format, which does not erase all existing data and a long option that does erase all existing data.

As a general rule,((refn|Not true for CMS file system<ref>((cite manual
 |      title = z/VM CMS Commands and Utilities Reference
 |    chapter = FORMAT
 | chapter-url = http://publib.boulder.ibm.com/infocenter/zvm/v5r4/index.jsp?topic=/com.ibm.zvm.v54.dmsb4/for.htm
 |    version = z/VM Version 5 Release 4
 |  publisher = IBM
 |       year = 2008
 |        url = http://publib.boulder.ibm.com/infocenter/zvm/v5r4/index.jsp?topic=/com.ibm.zvm.v54.dmsb4
 |         id = SC24-6073-03
 |      quote = When you do not specify either the RECOMP or LABEL option, the disk area is initialized by writing a device-dependent number of records (containing binary zeros) on each track. Any previous data on the disk is erased.
 |mode=cs2
 ))</ref> on a CMS minidisk, TSS VAM-formatted volume,<ref>((cite manual
 |     author = IBM
 |      title = IBM System/360 Time Sharing System System Logic Summary Program Logic Manual
 |    section = Virtual Access Methods
 |  publisher = IBM
 |        url = http://bitsavers.trailing-edge.com/pdf/ibm/360/tss/GY28-2009-2_Time_Sharing_System_System_Logic_Summary_PLM_Jun70.pdf
 |         id = GY28-2009-2
 |      quote = The direct access volumes, on which TSS/360 virtual organization data sets are stored, have fixed-length, page size data blocks. No key field is required. The record overflow feature is utilized to allow data blocks to span tracks, as required. The entire volume, with the current exception of part of the first cylinder, which is used for identification, is formatted into page size blocks.
 |       page = 56 (PDF 66)
 |mode=cs2
 ))</ref> z/OS Unix file systems<ref>((cite manual
 |       title = z/OS 2.4 File System Administration
 |          id = SC23-6887-40
 |     section = ioeagfmt
 | section-url = https://www.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R4sc236887/$file/ioea700_v2r4.pdf#page=142
 |       pages = 116–119
 |         url = https://www.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R4sc236887/$file/ioea700_v2r4.pdf
 |   publisher = IBM
 )) 
</ref> or VSAM<!-- Add AMS citation --> in IBM mainframes|group="lower-alpha")) formatting a disk by default leaves most if not all existing data on the disk medium; some or most of which might be recoverable with [[Privilege (computing)|privileged]]<ref group="lower-alpha">E.g., AMASPZAP in MVS</ref> or [[Data recovery|special tools]].<ref>((cite web
 |url          = https://www.linux.com/news/how-recover-lost-files-after-you-accidentally-wipe-your-hard-drive/
 |title        = How to recover lost files after you accidentally wipe your hard drive
 |last         = Hermans
 |first        = Sherman
 |date         = 28 August 2006
 |publisher    = Linux.com
 |access-date   = 28 November 2019
))</ref>  Special tools can remove user data by a single [[Data erasure|overwrite]] of all files and free space.<ref>((cite web
 |url          = http://www.infosecisland.com/blogview/16130-The-Urban-Legend-of-Multipass-Hard-Disk-Overwrite.html
 |title        = The Urban Legend of Multipass Hard Disk Overwrite and DoD 5220-22-M
 |last         = Smithson
 |first        = Brian
 |date         = 29 August 2011
 |publisher    = Infosec Island
 |access-date  = 22 November 2012
 |archive-date = 5 October 2018
 |archive-url  = https://web.archive.org/web/20181005045747/http://www.infosecisland.com/blogview/16130-The-Urban-Legend-of-Multipass-Hard-Disk-Overwrite.html
 |url-status   = dead
))</ref>

== History ==

A [[Block (data storage)|block]], a contiguous number of [[byte]]s, is the minimum unit of storage that is read from and written to a disk by a disk driver. The earliest disk drives had fixed block sizes (e.g. the [[IBM 350]] disk storage unit (of the late 1950s) block size was 100 six-bit characters) but starting with the [[IBM 1301|1301]]<ref>((cite web |url=http://www-03.ibm.com/ibm/history/exhibits/storage/storage_1301.html |title=IBM&nbsp;1301 disk storage unit |date=23 January 2003 |publisher=[[IBM]] |access-date=2010-06-24))</ref> IBM marketed subsystems that featured variable block sizes: a particular track could have blocks of different sizes. The disk subsystems and other [[direct access storage device]]s on the [[IBM System/360]] expanded this concept in the form of [[Count Key Data]] (CKD) and later [[Count Key Data#ECKD|Extended Count Key Data]] (ECKD); however the use of variable block size in HDDs fell out of use in the 1990s; one of the last HDDs to support variable block size was the IBM 3390 Model 9, announced May 1993.<ref>((cite web |url=http://www-03.ibm.com/ibm/history/exhibits/storage/storage_3390.html |title=IBM 3390 direct access storage device |date=23 January 2003 |publisher=[[IBM]]))</ref>

Modern hard disk drives, such as [[Serial attached SCSI]] (SAS)<ref group="lower-alpha">"The LBAs on a logical unit shall begin with zero and shall be contiguous up to the last logical block on the logical unit"., Information technology&nbsp;— Serial Attached SCSI - 2 (SAS-2), INCITS 457 Draft 2, May 8, 2009, chapter 4.1 Direct-access block device type model overview.</ref> and [[Serial ATA]] (SATA)<ref>ISO/IEC 791D:1994, AT Attachment Interface for Disk Drives (ATA-1), section 7.1.2</ref> drives, appear at their [[interface (computing)|interface]]s as a contiguous set of fixed-size blocks; for many years 512 bytes long but beginning in 2009 and accelerating through 2011, all major hard disk drive manufacturers began releasing hard disk drive platforms using the [[Advanced Format]] of 4096 byte logical blocks.<ref>((cite web |url=http://www.anandtech.com/show/2888 |title=Western Digital's Advanced Format: The 4K Sector Transition Begins |author-first=Ryan |author-last=Smith |date=2009-12-18 |publisher=[[Anandtech]]))</ref><ref>((cite web |url=http://www.seagate.com/tech-insights/advanced-format-4k-sector-hard-drives-master-ti/ |title=Transition to Advanced Format 4K Sector Hard Drives |publisher=[[Seagate Technology]]))</ref>

[[Floppy disk]]s generally only used fixed block sizes but these sizes were a function of the host's [[Operating System|OS]] and its interaction with its [[Floppy disk controller|controller]] so that a particular type of media (e.g., 5¼-inch DSDD) would have different block sizes depending upon the host OS and controller.

[[Optical disc]]s generally only use fixed block sizes.

== Disk formatting process ==

Formatting a disk for use by an operating system and its applications typically involves three different processes.<ref group="lower-alpha">Each process may involve multiple steps, and steps of different processes may be interleaved.</ref>

# Low-level formatting (i.e., closest to the hardware) marks the surfaces of the disks with markers indicating the start of a recording block (typically today called sector markers) and other information like block [[Cyclic redundancy check|CRC]] to be used later, in normal operations, by the [[disk controller]] to read or write data. This is intended to be the permanent foundation of the disk, and is often completed at the factory.
# [[Disk partitioning|Partitioning]] divides a disk into one or more regions, writing data structures to the disk to indicate the beginning and end of the regions. This level of formatting often includes checking for defective tracks or defective sectors.
# High-level formatting creates the [[file system]] format within a disk partition or a [[logical volume]].<ref name="Tanenbaum" /> This formatting includes the data structures used by the OS to identify the logical drive or partition's contents. This may occur during operating system installation, or when adding a new disk. [[List of file systems|Disk and distributed file system]] may specify an optional boot block, and/or various volume and directory information for the operating system.

=== Low-level formatting of floppy disks ===

The low-level format of floppy disks (and early hard disks) is performed by the disk drive's controller.

For a standard [[Floppy disk#microfloppy|1.44 MB floppy disk]], low-level formatting normally writes 18 [[Disk sector|sector]]s of 512 [[byte]]s to each of 160 tracks (80 on each side) of the floppy disk, providing 1,474,560 bytes of storage on the disk.

Physical sectors are actually larger than 512 bytes, as in addition to the 512 byte data field they include a sector identifier field, [[Cyclic redundancy check|CRC]] bytes (in some cases [[Error detection and correction|error correction bytes]]) and gaps between the fields. These additional bytes are not normally included in the quoted figure for overall storage capacity of the disk.

Different low-level formats can be used on the same [[Recording medium|media]]; for example, large records can be used to cut down on inter-record gap size.

Several [[freeware]], [[shareware]] and [[free software]] programs (e.g. [[GParted]], [[Fdformat|FDFORMAT]], NFORMAT, [[VGA-Copy]] and [[2M (DOS)|2M]]) allowed considerably more control over formatting, allowing the formatting of high-density 3.5" disks with a capacity up to 2 MB.

Techniques used include:

* head/track sector skew (moving the sector numbering forward at side change and track stepping to reduce mechanical delay),
* [[interleaving (disk storage)|interleaving]] sectors (to boost throughput by organizing the sectors on the track),
* increasing the number of sectors per track (while a normal 1.44 MB format uses 18 sectors per track, it is possible to increase this to a maximum of 21), and
* increasing the number of tracks (most drives could tolerate extension to 82 tracks: though some could handle more, others could jam).

[[Linux]] supports a variety of sector sizes,<ref>((Cite web|url=https://tools.ietf.org/doc/fdutils/Fdutils.html#Media-description|title = Fdutils))</ref> and [[DOS]] and [[Microsoft Windows|Windows]] support a large-record-size [[Distribution Media Format|DMF]]-formatted floppy format.<ref>((cite web
|url=http://support.microsoft.com/kb/120348
|title=Definition of Distribution Media Format (DMF)
|publisher=[[Microsoft Knowledge Base]]
|date=2007-01-19
|access-date=2011-10-16
|archive-url=https://web.archive.org/web/20110914232540/http://support.microsoft.com/kb/120348
|archive-date=2011-09-14
|url-status=dead))</ref>

After establishing the structure of tracks, a formatter also needs to fill the entire floppy and look for [[bad sector]]s. Traditionally, the physical sectors were initialized with a fill value of <code>0xF6</code> as per the INT&nbsp;1Eh's [[Disk Parameter Table]] (DPT)<!-- TBD: IIRC this resembles a bit pattern optimized for MFM controllers --> during format on IBM compatible machines. This value is also used on the [[Atari Portfolio]]. [[CP/M]] 8-inch floppies typically came pre-formatted with a value of <code>0xE5</code>,<ref name="Schulman_1994_Undocumented-DOS">((cite book |author-first1=Andrew |author-last1=Schulman |author-first2=Ralf D. |author-last2=Brown |author-link2=Ralf D. Brown |author-first3=David |author-last3=Maxey |author-first4=Raymond J. |author-last4=Michels |author-first5=Jim |author-last5=Kyle |title=Undocumented DOS: A programmer's guide to reserved MS-DOS functions and data structures - expanded to include MS-DOS 6, Novell DOS and Windows 3.1 |publisher=[[Addison Wesley]] |edition=2 |date=1994 |orig-year=November 1993<!-- first printing --> |isbn=0-201-63287-X |url=https://archive.org/details/undocumenteddosp00andr_0 )) (xviii+856+vi pages, 3.5"-floppy) Errata: [https://web.archive.org/web/20190417215556/http://www.cs.cmu.edu/afs/cs/user/ralf/pub/books/UndocumentedDOS/errata.ud2][https://web.archive.org/web/20190417212906/https://www.pcjs.org/pubs/pc/programming/Undocumented_DOS/#errata-2nd-edition]</ref> and by way of [[Digital Research]] this value was also used on [[Atari ST]] and some [[Amstrad]] formatted floppies.<ref group="lower-alpha" name="NB_Magic_E5"/> Amstrad otherwise used <code>0xF4</code> as a fill value.

=== Low-level formatting (LLF) of hard disks ===
[[File:IBM PC XT 10 meg MFM low level format.jpg|thumb|Low-level format of a 10-megabyte [[IBM Personal Computer XT|IBM PC XT]] hard drive]]

Hard disk drives prior to the 1990s typically had a separate [[disk controller]] that defined how data was encoded on the media. With the media, the drive and/or the controller possibly procured from separate vendors, users were often able to perform low-level formatting. Separate procurement also had the potential of incompatibility between the separate components such that the subsystem would not reliably store data.<ref group="lower-alpha">This problem became common in PCs where users used RLL controllers with MFM drives; [https://web.archive.org/web/20180609221448/http://webpages.charter.net/dperr/diskguid.txt "MFM drives should not be used on RLL controllers.".]</ref>

User-instigated low-level formatting (LLF) of [[hard disk drives]] was common for [[minicomputer]] and [[personal computer]] systems until the 1990s. [[IBM]] and other mainframe system vendors typically supplied their hard disk drives (or media in the case of removable media HDDs) with a low-level format. Typically this involved subdividing each track on the disk into one or more blocks which would contain the user data and associated control information. Different computers used different block sizes and IBM notably used [[Count Key Data|variable block sizes]] but the popularity of the IBM PC caused the industry to adopt a standard of 512 user data bytes per block by the middle 1980s.

Depending upon the system, low-level formatting was generally done by an operating system utility. IBM compatible PCs used the BIOS, which is invoked using the MS-DOS [[DEBUG (DOS Command)|debug]] program, to transfer control to a routine hidden at different addresses in different BIOSes.<ref>[http://support.microsoft.com/kb/60089 Using DEBUG to Start a Low-Level Format], Microsoft</ref>

==== Transition away from LLF ====

Starting in the late 1980s, driven by the volume of IBM compatible PCs, HDDs became routinely available pre-formatted with a compatible low-level format. At the same time, the industry moved from [[Hard disk drive interface#BSDI|''historical (dumb) bit serial interfaces'']] to modern (intelligent) [[Hard disk drive interface#BSI|''bit serial interfaces'']] and [[Hard disk drive interface#WSI|''word serial interfaces'']] wherein the low-level format was performed at the factory.<ref>((cite web|publisher=The NOSPIN Group, Inc.|url=http://freepctech.com/pc/001/007.shtml|title=Low level formatting an IDE hard drive|website=FreePCTech.com|archive-url=https://web.archive.org/web/20120716043736/http://freepctech.com/pc/001/007.shtml|archive-date=July 16, 2012|access-date=December 24, 2003))</ref><ref>((cite web|website=The PC Guide. Site Version: 2.2.0 - Version Date: April 17, 2001|url=http://www.pcguide.com/ref/hdd/geom/formatUtilities-c.html|title=Low-Level Format, Zero-Fill and Diagnostic Utilities|access-date=May 24, 2007|archive-url=https://web.archive.org/web/20190103014814/http://www.pcguide.com/ref/hdd/geom/formatUtilities-c.html|archive-date= January 3, 2019))</ref>  Accordingly, it is not possible for an end user to low-level format a modern hard disk drive.

=== Modern disks: reinitialization ===
Modern hard drives can no longer perform post-production LLF, i.e. to re-establish the basic layout of "tracks" and "blocks" on the recording surface. ''Reinitialization'' refers to processes that return a disk to a factory-like configuration: no data, no partitioning, all blocks available to use.

==== Command-set support ====
SCSI provides a ((tt|Format Unit)) command. This command performs the needed certification step to weed out [[bad sector]]s and has the ability to change sector size. The command-line sg_format program may be used to issue the command.<ref>((man|8|sg_format|Linux))</ref> A variety of sector sizes may be chosen, but are not available on all devices: 512, 520, 524, 528, 4096, 4112, 4160, and 4224-byte sectors.<ref>[http://www.seagate.com/docs/pdf/whitepaper/tp595_building_faster_more_flexible_infrastructure.pdf Seagate SAS drives] ((webarchive |url=https://web.archive.org/web/20101129180307/http://www.seagate.com/docs/pdf/whitepaper/tp595_building_faster_more_flexible_infrastructure.pdf |date=2010-11-29))</ref> Although the SCSI command provides many options, even resizing, it does not touch on the track layer where low-level format happens.<ref>((cite web |title=INCITS 506-202x - Information technology - SCSI Block Commands - 4 (SBC-4) draft revision 22 |url=https://standards.incits.org/apps/group_public/download.php/124286/livelink |access-date=22 May 2023 |date=15 September 2020))</ref>

ATA does not expose a low-level format functionality, but they allow the sector size to be changed via ((tt|SET SECTOR CONFIGURATION)) (((tt|--set-sector-size)) in <code>[[hdparm]]</code>). (Consumer drives usually only support 512 and [[Advanced Format|4096-byte sector]]s.) Although sector-size change may scramble data, it is not a safe way of erasing data, nor is any certification done. ATA offers a separate ((tt|SECURITY ERASE)) (((tt|--security-erase)) in <code>[[hdparm]]</code>) command for erasure.<ref>((man|8|hdparm|Linux))</ref>

[[NVMe]] drives have a standard method of formatting, available in, for example, the Linux command-line program ((tt|nvme format)). Sector size change and secure erase options are available.<ref>((man|1|nvme-format|Linux))</ref> Note that NVMe drives are generally solid-state, making this "track" distinction useless.

[[Seagate Technology]] drives offer a [[TTL serial]] debugging console.<ref>((cite web |title=Seagate Serial Talk ((!)) OS/2 Museum |url=https://www.os2museum.com/wp/seagate-serial-talk/))</ref> Among other things, the console can format the "system" and "user" partitions while performing defect checks (re-initialization over pre-established logical blocks) and modify track parameters (managing the ''real'' low-level format).<ref>((cite web |title=F3 Serial Port Diagnostics |url=https://dokumen.tips/documents/f3-serialport-diagnostics.html?page=1)) older version available from </ref>

==== Disk-filling ====
When the hard drive's built-in reinitialization function (see above) is unavailable due to driver or system limitations, it is possible to fill the entire disk instead. On older hard drives without [[bad sector]] management,<ref>((cite web |title=BadBlockHowto – smartmontools |url=https://www.smartmontools.org/wiki/BadBlockHowto |website=www.smartmontools.org))</ref> a program will also need to check for any damaged sectors and try to spare them out. On newer drives with defect management, reallocated sectors may be left unerased, whereas the built-in re-initialization function will erase them.<ref name="Secure Deletion" />

In modern times, it is most common to fill hard drives with value of <code>0x00</code>. One popular method for performing this zero-fill operation on a hard disk is by writing zero-value bytes to the drive using the Unix [[dd (Unix)|dd]] utility with the [[/dev/zero]] stream as the input file and the drive itself (or a specific partition) as the output file.<ref>((cite web|url=http://www.myfixlog.com/fix.php?fid=58|title=How to Securely Erase (Wipe) a Hard Drive for Free with DD|website=myfixlog.com|archive-url=https://web.archive.org/web/20160418143615/http://www.myfixlog.com/fix.php?fid=58|archive-date=April 18, 2016))</ref> This command may take many hours to complete, and will erase all files and file systems.

A value of <code>0xFF</code> is used on flash disks to reduce [[Program-erase cycle|wear]] . The latter value is typically also the default value used on ROM disks (which cannot be reformatted). Some advanced tools allow configuring the fill value.<ref group="lower-alpha" name="NB_Format_Wipe"/>

Zero-filling a drive is not a secure method of preparing a drive for use with an encrypted filesystem. Doing so voids the [[deniable encryption|plausible deniability of the process]], as the encrypted areas (indistinguishable from random without a key, unless the cipher is compromised) will stand out among zero blocks. The correct technique is to zero-fill inside a temporary encrypted layer than discard the key and layer setup. ([[/dev/urandom]] provides similar safety, but tends to be slow.)<ref>[http://www.globallinuxsecurity.pro/quickly-fill-a-disk-with-random-bits-without-dev-urandom/ Quickly fill a disk with random bits]</ref>

==== Confusion ====
((More citations needed section|date=July 2009))
The present ambiguity in the term ''low-level format'' seems to be due to both inconsistent documentation on web sites and the belief by many users that any process below a high-level (file system) format must be called a ''low-level'' format. Since much of the low-level formatting process can today only be performed at the factory, various drive manufacturers describe reinitialization software as LLF utilities on their web sites. Since users generally have no way to determine the difference between a complete LLF and ''reinitialization'' (they simply observe running the software results in a hard disk that must be high-level formatted), both the misinformed user and mixed signals from various drive manufacturers have perpetuated this error.

Note: whatever possible misuse of such terms may exist, many sites do make such ''reinitialization'' utilities available (possibly as bootable floppy diskette or CD image files), to both overwrite every byte ''and'' check for damaged sectors on the hard disk.

=== Partitioning ===
((main|Disk partitioning))

Partitioning is the process of writing information into blocks of a storage device or medium to divide the device into several sub-devices, each of which is treated by the operating system as a separate device and, in some cases, to allow an operating system to be booted from the device.

On [[MS-DOS]], [[Microsoft Windows]], and UNIX-based operating systems (such as [[BSD]], [[Linux]] and [[macOS]]) this is normally done with a [[partition editor]], such as [[fdisk]], [[GNU Parted]], or [[Disk Utility]]. These operating systems support multiple partitions.

Floppy disks are not partitioned; however depending upon the OS they may require volume information in order to be accessed by the OS.

[[Partition editor]]s and ICKDSF today do not handle low-level functions for HDDs and optical disc drives such as writing timing marks, and they cannot reinitialize a modern disk that has been degaussed or otherwise lost the factory formatting.

IBM operating systems derived from [[CP-67]], e.g., [[z/VM]], maintain partitioning information for [[VM (operating system)#Minidisks|minidisks]] externally to the drive.

=== High-level formatting ===

High-level formatting is the process of setting up an empty file system on a disk partition or a [[logical volume]] and for PCs, installing a [[boot sector]].<ref name="Tanenbaum" /> This is often a fast operation, and is sometimes referred to as ''quick formatting''.

Formatting an entire logical drive or partition may optionally scanned for defects, which may take considerable time.

In the case of floppy disks, both high- and low-level formatting are customarily performed in one pass by the disk formatting software. Eight-inch floppies typically came low-level formatted and were filled with a format filler value of <code>0xE5</code>.<ref name="Schulman_1994_Undocumented-DOS"/><ref group="lower-alpha" name="NB_Magic_E5"/> Since the 1990s, most 5.25-inch and 3.5-inch floppies have been shipped pre-formatted from the factory as DOS [[FAT12]] floppies.

In current IBM mainframe operating systems derived from [[OS/360]] and [[DOS/360]], such as [[z/OS]] and [[z/VSE]], formatting of drives is done by the INIT command of the [[ICKDSF]] utility.<ref>((Cite web |url=http://publibz.boulder.ibm.com/epubs/pdf/ick4020f.pdf |title=Device Support Facilities User's Guide and Reference |access-date=2010-12-27 |archive-date=2021-12-09 |archive-url=https://web.archive.org/web/20211209100904/http://publibz.boulder.ibm.com/epubs/pdf/ick4020f.pdf |url-status=dead ))</ref> These OSs support only a single partition per device, called a volume. The ICKDSF functions include writing a Record 0 on every track, writing [[Initial Program Load|IPL]] text, creating a volume label, creating a [[Volume Table of Contents]] (VTOC) and, optionally, creating a VTOC index (VTOCIX); high level formatting may also be done as part of allocating a file, by a utility specific to a file system or, in some older access methods, on the fly as new data are written. In z/OS Unix System Services, there are three distinct levels of high-level formatting:
*Initializing a volume with ICKDSF
*Initializing a [[VSAM]] Linear Data Set (LDS) as part of allocating it on the volume with Access Method Services (IDCAMS) DEFINE
*Initializing a [[zFS (z/OS file system)|zFS]] aggregate in the LDS using ioeagfmt.

In IBM operating systems derived from [[CP-67]], formatting a volume initializes track 0 and a dummy VTOC. Guest operating systems are responsible for formatting [[minidisk (VM)|minidisks]]; the CMS FORMAT command formats a [[CMS file system]] on a CMS minidisk.

== Host protected area ==
((main|Host protected area))

The host protected area, sometimes referred to as hidden protected area, is an area of a [[hard drive]] that is high-level formatted such that the area is not normally visible to its [[operating system]] (OS).

== Reformatting ((anchor|REFORMAT)) ==<!-- incoming links -->
Reformatting is a [[#HIGH|high-level formatting]] performed on a functioning disk drive to free the medium of its contents. Reformatting is unique to each operating system because what actually is done to existing data varies by OS. The most important aspect of the process is that it frees disk space for use by other data. To actually "erase" everything requires overwriting each block of data on the medium; something that is not done by many high-level formatting utilities.

Reformatting often carries the implication that the operating system and all other software will be reinstalled after the format is complete. Rather than fixing an installation suffering from malfunction or security compromise, it may be necessary to simply reformat everything and start from scratch. Various colloquialisms exist for this process, such as "wipe and reload", "nuke and pave", "reimage", etc. However, reformatting a drive containing only user data does not require reinstallation of the OS.

== Formatting ==

=== DOS, OS/2 and Windows ===
[[Image:Unformat.gif|thumb|MS-DOS 6.22a FORMAT /U switch failing to overwrite content of partition]]

''format command'': Under [[MS-DOS]], [[PC DOS]], [[OS/2]] and [[Microsoft Windows]], disk formatting can be performed by the <code>[[format (command)|format]]</code> [[command (computing)|command]]. The <code>format</code> program usually asks for confirmation beforehand to prevent accidental removal of data, but some versions of DOS have an undocumented <code>/AUTOTEST</code> option; if used, the usual confirmation is skipped and the format begins right away. The WM/FormatC [[Macro virus (computing)|macro virus]] uses this command to format drive C: as soon as a document is opened.

''Unconditional format'': There is also the <code>/U</code> parameter that performs an ''unconditional'' format which under most circumstances overwrites the entire partition,<ref>((cite web |url = http://www.mdgx.com/secrets.htm#FORMAT-U |title = AXCEL216 / MDGx MS-DOS Undocumented + Hidden Secrets |access-date = 2008-06-07))</ref> preventing the recovery of data through software. Note however that the <code>/U</code> switch only works reliably with floppy diskettes (see image to the right). Technically because unless <code>/Q</code> is used, floppies are always low level formatted in addition to high-level formatted. Under certain circumstances with hard drive partitions, however, the <code>/U</code> switch merely prevents the creation of <code>[[unformat (command)|unformat]]</code> information in the partition to be formatted while otherwise leaving the partition's contents entirely intact (still on disk but marked deleted). In such cases, the user's data remain ripe for recovery with specialist tools such as [[EnCase]] or [[disk editor]]s. Reliance upon <code>/U</code> for secure overwriting of hard drive partitions is therefore inadvisable, and purpose-built tools such as [[DBAN]] should be considered instead.

''Overwriting'': In Windows Vista and upwards the non-quick format will overwrite as it goes. Not the case in Windows XP and below.<ref>
  ((cite web
    | url = http://support.microsoft.com/kb/941961
    | title = MSKB941961: Change in the behavior of the format command in Windows Vista
    | publisher = [[Microsoft Corporation]]
    | quote = The format command behavior has changed in Windows Vista. By default in Windows Vista, the format command writes zeros to the whole disk when a full format is performed. In Windows XP and in earlier versions of the Windows operating system, the format command does not write zeros to the whole disk when a full format is performed.
    | date = 2009-02-23
    | access-date = 2012-10-24
  ))
</ref>

''OS/2'': Under OS/2, format will overwrite the entire partition or logical drive if the <code>/L</code> parameter is used, which specifies a ''long'' format. Doing so enhances the ability of [[CHKDSK]] to recover files.

=== Unix-like operating systems ===

High-level formatting of disks on these systems is traditionally done using the <code>[[mkfs]]</code> command. On Linux (and potentially other systems as well) <code>mkfs</code> is typically a wrapper around filesystem-specific commands which have the name <code>mkfs''.fsname''</code>, where ''fsname'' is the name of the filesystem with which to format the disk.<ref>((cite web |url = http://linux.die.net/man/8/mkfs |title = mkfs(8) - Linux man page |access-date = 2010-04-25))</ref> Some filesystems which are not supported by certain implementations of <code>mkfs</code> have their own manipulation tools; for example [[Ntfsprogs]] provides a format utility for the [[NTFS]] filesystem.

Some Unix and Unix-like operating systems have higher-level formatting tools, usually for the purpose of making disk formatting easier and/or allowing the user to partition the disk with the same tool. Examples include [[GNU Parted]] (and its various GUI frontends such as [[GParted]] and the [[KDE Partition Manager]]) and the [[Disk Utility]] application on [[Mac OS X]].

== Recovery of data from a formatted disk ==

As in file deletion by the operating system, data on a disk are not fully erased during every high-level format. Instead, the area on the disk containing the data is merely marked as available, and retains the old data until it is overwritten. If the disk is formatted with a different file system than the one which previously existed on the partition, some data may be overwritten that wouldn't be if the same file system had been used. However, under some file systems (e.g., NTFS, but not FAT), the file indices (such as $MFTs under NTFS, inodes under ext2/3, etc.) may not be written to the same exact locations. And if the partition size is increased, even FAT file systems will overwrite more data at the beginning of that new partition.

From the perspective of preventing the recovery of sensitive data through recovery tools, the data must be completely overwritten (every sector), either by a separate tool, or during formatting. Data are destroyed in DOS, OS/2, and Windows when the '''/L''' (long) option is used on format and always for a [[Data set (IBM mainframe)#Partitioned datasets|Partitioned Data Set (PDS)]] in [[MVS]] and for newer file systems on IBM mainframes.

((main|data erasure))
It is disputed whether one pass of zero-fill is enough to destroy sensitive data on older (until 1990s) magnetic storage: Gutmann (known for his 35-pass [[Gutmann method]]) claims that [[magnetic force microscopy]] may be able to "see" old bits on a floppy,<ref name="Gutmann">Gutmann, Peter. (July 22–25, 1996) ''[https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html Secure Deletion of Data from Magnetic and Solid-State Memory.]'' University of Auckland Department of Computer Science. Epilogue section.</ref> but the sources he cited does not prove such. Random fill is believed to be stronger than a fixed pattern fill.<ref>((cite web|date=2003|title=Can Intelligence Agencies Recover Overwritten Data?|author=Daniel Feenberg|url=http://www.nber.org/sys-admin/overwritten-data-gutmann.html|access-date=2007-12-10))</ref> One pass of zero fill is sufficient to prevent [[data remanence]], according to NIST (2014) and Wright et al (2008).<ref>((cite conference
 | first = Craig | last = Wright
 |author2=Kleiman, Dave
 |author2-link=Dave Kleiman
 |author3=Shyaam, Sundhar R.S.
 | conference = Information Systems Security ICISS 2008 | title = Overwriting Hard Drive Data: The Great Wiping Controversy | series = Lecture Notes in Computer Science | publisher = Springer Berlin / Heidelberg | isbn = 978-3-540-89861-0 | doi = 10.1007/978-3-540-89862-7_21
 | pages = 243–257 |date=December 2008
| volume = 5352 ))</ref><ref>((cite tech report
 | url = https://csrc.nist.gov/publications/detail/sp/800-88/rev-1/final
 | title = Special Publication 800-88 Rev. 1: Guidelines for Media Sanitization
 | publisher = [[National Institute of Standards and Technology|NIST]]
 | date = December 2014 | doi = 10.6028/NIST.SP.800-88r1
 | access-date = 2018-06-26
| last1 = Kissel
 | first1 = Richard
 | last2 = Regenscheid
 | first2 = Andrew
 | last3 = Scholl
 | first3 = Matthew
 | last4 = Stine
 | first4 = Kevin
 | doi-access = free
 ))</ref> The ''[[Secure Erase]]'' option built into hard drives is considered trustworthy,<ref name="Secure Deletion">((cite web
| url=http://www.upenn.edu/computing/security/privacy/data_clear.php
| title=Secure Data Deletion
| date=June 7, 2012
| access-date=9 December 2013
))</ref><ref>((cite web
|url=http://tinyapps.org/docs/wipe_drives_hdparm.html
|title=ATA Secure Erase (SE) and hdparm
)) Created: 2011.02.21, updated: 2013.04.02.</ref> with the caveat that early [[solid state drives]] are known to mis-implement the function.<ref name="Wei2011">((cite q | Q115346857
| journal            = FAST'11: Proceedings of the 9th USENIX conference on File and storage technologies
| access-date        = 2018-01-08
| ref                = ((sfnref|Wei|2011))
))</ref>

[[Degaussing]] is effective without controversy; however, this may render the drive [[Degaussing#Irreversible damage to some media types|unusable]].<ref name="Secure Deletion" />

== See also ==
* [[Data erasure]]
* [[Data recovery]]
* [[Data remanence]]
* [[Drive mapping]]
* [[Comparison of file systems]]

== Notes ==
((Notelist|refs=
<ref group="lower-alpha" name="NB_Magic_E5">The fact that 8-inch CP/M floppies came pre-formatted with a filler value of <code>0xE5</code> is the reason why the value of <code>0xE5</code> has a special meaning in directory entries in [[FAT12]], [[FAT16]] and [[FAT32]] file systems. This allowed [[86-DOS]] to use 8-inch floppies out of the box or with only the FAT initialized.<!-- <ref name="Schulman_1994_Undocumented-DOS"/> --></ref>
<ref group="lower-alpha" name="NB_Format_Wipe">One utility providing an option to specify the desired fill value for hard disks is DR-DOS' FDISK&nbsp;R2.31 with its optional wipe parameter <code>/W:246</code> (for a fill value of <code>0xF6</code>). In contrast to other [[FDISK]] utilities, DR-DOS FDISK is not only a partitioning tool, but can also format freshly created partitions as [[#FAT12|FAT12]], [[#FAT16|FAT16]] or [[#FAT32|FAT32]]. This reduces the risk of accidentally formatting the wrong volume.</ref>
))

== References ==
((reflist|refs=
))

== External links ==
* [https://technet.microsoft.com/en-us/library/cc750198.aspx Windows NT Workstation Resource Kit, Chapter 17 - Disk and File System Basics], section "Formatting Hard Disks and Floppy Disks"
* [http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html ''Secure Deletion of Data from Magnetic and Solid-State Memory''] by Peter Gutmann
* [http://support.microsoft.com/?kbid=302686 ''Differences between a Quick format and a regular format during a "clean" installation of Windows XP''] from Microsoft Help and Support
* [http://support.microsoft.com/?scid=kb%3Ben-us%3B255867&x=17&y=15 support.microsoft.com — How to Use the Fdisk Tool and the Format Tool to Partition or Repartition a Hard Disk]
* [https://technet.microsoft.com/en-us/library/cc512587.aspx ''Help: I Got Hacked. Now What Do I Do?'']—Microsoft Tech Net: Why you should wipe a compromised drive to the bare metal. Article by Jesper M. Johansson, Ph.D., CISSP, MCSE, MCP+I

[[Category:Rotating disc computer storage media|Formatting of disks]]
[[Category:File system management]]

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