Editing Hard disk drive (section)

== Capacity ==
[[File:Seagate Barracuda IMG 7901.jpg|thumb|Two [[Seagate Barracuda]] drives from 2003 and 2009, respectively 160&nbsp;GB and 1&nbsp;TB. {{As of|2025}}, Seagate offers capacities up to 36&nbsp;TB.]]
[[File:MSATA SSD vs. 2.5" SATA drive.JPG|thumb|mSATA SSD on top of a 2.5-inch hard drive]]
The highest-capacity HDDs shipping commercially {{as of|2025|lc=on}} are 36&nbsp;TB.<ref name="x-mozaic">{{cite web | url=https://www.seagate.com/products/enterprise-drives/exos-x/x-mozaic/ | title=Exos Mozaic 3+ |website= Seagate US |access-date=2025-02-17}}</ref>{{fv|reason=Shows that 36 TB drives exist but doesn't establish this is the highest available capacity|date=January 2025}}  The capacity of a hard disk drive, as reported by an operating system to the end user, is smaller than the amount stated by the manufacturer for several reasons, e.g. the operating system using some space, use of some space for data redundancy, space use for file system structures. Confusion of [[decimal prefix]]es and [[binary prefix]]es can also lead to errors.

=== Calculation ===
Modern hard disk drives appear to their host controller as a contiguous set of logical blocks, and the gross drive capacity is calculated by multiplying the number of blocks by the block size. This information is available from the manufacturer's product specification, and from the drive itself through use of operating system functions that invoke low-level drive commands.<ref name="SAS" /><ref name="SATA" /> Older IBM and compatible drives, e.g. [[IBM 3390]] using the [[Count key data|CKD]] record format, have variable length records; such drive capacity calculations must take into account the characteristics of the records. Some newer DASD simulate CKD, and the same capacity formulae apply.

The gross capacity of older sector-oriented HDDs is calculated as the product of the number of [[cylinder (disk drive)|cylinders]] per recording zone, the number of bytes per sector (most commonly 512), and the count of [[Zone bit recording|zones]] of the drive.{{citation needed|date=April 2016}} Some modern SATA drives also report [[cylinder-head-sector]] (CHS) capacities, but these are not physical parameters because the reported values are constrained by historic operating system interfaces. The C/H/S scheme has been replaced by [[logical block addressing]] (LBA), a simple linear addressing scheme that locates blocks by an integer index, which starts at LBA 0 for the first block and increments thereafter.<ref>{{cite web | url = http://www.idema.org/wp-content/plugins/download-monitor/download.php?id=1223 | title = LBA Count for Disk Drives Standard (Document LBA1-03) | date = June 15, 2009 | access-date = February 14, 2016 | publisher = [[IDEMA]] | format = PDF | archive-url = https://web.archive.org/web/20160222082703/http://www.idema.org/wp-content/plugins/download-monitor/download.php?id=1223 | archive-date = February 22, 2016 | url-status = live }}</ref> When using the C/H/S method to describe modern large drives, the number of heads is often set to 64, although a typical modern hard disk drive has between one and four platters. In modern HDDs, spare capacity for [[Internal hard-drive defect management|defect management]] is not included in the published capacity; however, in many early HDDs, a certain number of sectors were reserved as spares, thereby reducing the capacity available to the operating system. Furthermore, many HDDs store their firmware in a reserved service zone, which is typically not accessible by the user, and is not included in the capacity calculation.

For [[RAID]] subsystems, [[data integrity]] and fault-tolerance requirements also reduce the realized capacity. For example, a RAID&nbsp;1 array has about half the total capacity as a result of data mirroring, while a RAID&nbsp;5 array with {{mvar|n}} drives loses {{mvar|1/n}} of capacity (which equals to the capacity of a single drive) due to storing parity information. RAID subsystems are multiple drives that appear to be one drive or more drives to the user, but provide fault tolerance. Most RAID vendors use [[checksum]]s to improve data integrity at the block level. Some vendors design systems using HDDs with sectors of 520 bytes to contain 512 bytes of user data and eight checksum bytes, or by using separate 512-byte sectors for the checksum data.<ref name="AutoMK-38" />

Some systems may use [[Host protected area|hidden]] [[disk partitioning|partitions]] for system recovery, reducing the capacity available to the end user without knowledge of [[List of disk partitioning software|special disk partitioning utilities]] like [[diskpart]] in [[Windows]].<ref>{{Cite journal |last1=Gupta |first1=Mayank R. |last2=Hoeschele |first2=Michael D. |last3=Rogers |first3=Marcus K. |date=2006 |title=Hidden Disk Areas: HPA and DCO |url=https://www.utica.edu/academic/institutes/ecii/publications/articles/EFE36584-D13F-2962-67BEB146864A2671.pdf |journal=International Journal of Digital Evidence |volume=5 |issue=1}}</ref>

=== Formatting ===
{{Main|Disk formatting}}
Data is stored on a hard drive in a series of logical blocks.  Each block is delimited by markers identifying its start and end, error detecting and correcting information, and space between blocks to allow for minor timing variations.  These blocks often contained 512 bytes of usable data, but other sizes have been used.  As drive density increased, an initiative known as [[Advanced Format]] extended the block size to 4096 bytes of usable data, with a resulting significant reduction in the amount of disk space used for block headers, error-checking data, and spacing.

The process of initializing these logical blocks on the physical disk platters is called ''low-level formatting'', which is usually performed at the factory and is not normally changed in the field.<ref>{{cite web|url=http://www.pcguide.com/ref/hdd/geom/formatLow-c.html|title=Low-Level Formatting|access-date=June 28, 2010|archive-url=https://web.archive.org/web/20170604064402/http://www.pcguide.com/ref/hdd/geom/formatLow-c.html|archive-date=June 4, 2017|url-status=dead}}</ref> ''High-level formatting'' writes data structures used by the operating system to organize data files on the disk. This includes writing [[Disk partitioning|partition]] and [[file system]] structures into selected logical blocks. For example, some of the disk space will be used to hold a directory of disk file names and a list of logical blocks associated with a particular file.

Examples of partition mapping scheme include [[master boot record]] (MBR) and [[GUID Partition Table]] (GPT). Examples of data structures stored on disk to retrieve files include the [[File Allocation Table]] (FAT) in the [[DOS]] file system and [[inode]]s in many [[UNIX]] file systems, as well as other operating system data structures (also known as [[metadata]]). As a consequence, not all the space on an HDD is available for user files, but this system overhead is usually small compared with user data.

=== Units ===
{{See also|Binary prefix#Disk drives|l1=Binary prefix § disk drives}}

{| class="wikitable floatcenter" style="width: 60%; margin-left: 1.5em;"
|+ Decimal and binary [[unit prefix]]es interpretation<ref name="SeagateStorageGuide" /><ref name="WD2" />
! colspan=2 rowspan=2 | Capacity advertised by manufacturers{{Efn|name="units-decimal"|Expressed using [[SI prefix#List of SI prefixes|decimal&nbsp;multiples]]}}
! colspan=2 rowspan=2 | Capacity expected by some consumers{{Efn|name="units-binary"|Expressed using [[binary prefix|binary&nbsp;multiples]]}}
! colspan=2 | Reported capacity
|-
! rowspan=2 | [[Windows]]{{Efn|name="units-binary"}}
! rowspan=2 | [[macOS]] ver 10.6+{{Efn|name="units-decimal"}}
|-
! With prefix
! Bytes
! Bytes
! Diff.
|-
| style="text-align:right;"| 100&nbsp;[[Gigabyte|GB]]
| style="text-align:right;"| 100,000,000,000
| style="text-align:right;"| 107,374,182,400
| style="text-align:right;"| 7.37%
| style="text-align:right;"| 93.1&nbsp;GB
| style="text-align:right;"| 100&nbsp;GB
|-
| style="text-align:right;"| 1&nbsp;[[Terabyte|TB]]
| style="text-align:right;"| 1,000,000,000,000
| style="text-align:right;"| 1,099,511,627,776
| style="text-align:right;"| 9.95%
| style="text-align:right;"| 931&nbsp;GB
| style="text-align:right;"| 1,000&nbsp;GB, 1,000,000&nbsp;MB
|}

In the early days of computing, the total capacity of HDDs was specified in seven to nine decimal digits frequently truncated with the idiom ''millions''.<ref name="AutoMK-26" /><ref name="1301Ref" />
By the 1970s, the total capacity of HDDs was given by manufacturers using [[SI]] decimal prefixes such as [[megabyte]]s (1&nbsp;MB&nbsp;= 1,000,000&nbsp;bytes), [[gigabyte]]s (1&nbsp;GB&nbsp;= 1,000,000,000&nbsp;bytes) and [[terabyte]]s (1&nbsp;TB&nbsp;= 1,000,000,000,000 bytes).<ref name="SeagateStorageGuide" /><ref name="AutoMK-27" /><ref name="AutoMK-28" /><ref name="AutoMK-29" /> However, capacities of [[main memory|memory]] are usually quoted using a [[binary prefixes|binary interpretation]] of the prefixes, i.e. using powers of 1024 instead of 1000.

Software reports hard disk drive or memory capacity in different forms using either decimal or binary prefixes. The [[Microsoft Windows]] family of operating systems uses the binary convention when reporting storage capacity, so an HDD offered by its manufacturer as a 1&nbsp;TB drive is reported by these operating systems as a 931&nbsp;GB HDD. [[Mac OS X]] 10.6 ("[[Mac OS X Snow Leopard|Snow Leopard]]") uses decimal convention when reporting HDD capacity.<ref name="Apple" /> The default behavior of the {{Mono|df}} [[command-line utility]] on Linux is to report the HDD capacity as a number of 1024-byte units.<ref>{{cite web
 | url = http://linux.die.net/man/1/df
 | title = df(1) – Linux man page
 | access-date = July 18, 2015
 | website = linux.die.net
 | archive-url = https://web.archive.org/web/20150718060713/http://linux.die.net/man/1/df
 | archive-date = July 18, 2015
 | url-status = live
 }}</ref>

The difference between the decimal and binary prefix interpretation caused some consumer confusion and led to class action suits [[Binary prefix#Legal disputes|against HDD manufacturers]]. The plaintiffs argued that the use of decimal prefixes effectively misled consumers, while the defendants denied any wrongdoing or liability, asserting that their marketing and advertising complied in all respects with the law and that no class member sustained any damages or injuries.<ref name="WDSettle" /><ref name="AutoMK-30" /><ref name="AutoMK-31" />  In 2020, a California court ruled that use of the decimal prefixes with a decimal meaning was not misleading.<ref>{{Cite web|url=https://www.courthousenews.com/wp-content/uploads/2020/01/flashdrives.pdf|title=Order granted motion to dismiss amended complaint without leave to amend, 22 January 2020}}</ref>

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