Editing Hard disk drive (section)

=== 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 The C/H/S scheme has been replaced by [[logical block addressing]] (LBA), a simple linear addressing scheme that locates blocks by an integer index, which starts at LBA 0 for the first block and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>