Editing Hard disk drive (section)

== Form factors ==
{{main|List of disk drive form factors}}
[[File:SixHardDriveFormFactors.jpg|thumb|right|8-, 5.25-, 3.5-, 2.5-, 1.8- and 1-inch HDDs, together with a ruler to show the size of platters and read-write heads]]
[[File:5.25 inch MFM hard disk drive.JPG|thumb|A newer 2.5-inch (63.5&nbsp;mm) 6,495&nbsp;MB HDD compared to an older 5.25-inch full-height 110&nbsp;MB HDD]]

IBM's first hard disk drive, the [[IBM&nbsp;350]], used a stack of fifty 24-inch platters, stored 3.75 MB of data (approximately the size of one modern digital picture), and was of a size comparable to two large refrigerators. In 1962, [[IBM]] introduced its [[IBM 1311|model 1311]] disk, which used six 14-inch (nominal size) platters in a removable pack and was roughly the size of a washing machine. This became a standard platter size for many years, used also by other manufacturers.<ref name="AutoMK-39" /> The [[IBM 2314]] used platters of the same size in an eleven-high pack and introduced the "drive in a drawer" layout, sometimes called the "pizza oven", although the "drawer" was not the complete drive. Into the 1970s, HDDs were offered in standalone cabinets of varying dimensions containing from one to four HDDs.

Beginning in the late 1960s, drives were offered that fit entirely into a chassis that would mount in a [[19-inch rack]]. Digital's [[RK05]] and RL01 were early examples using single 14-inch platters in removable packs, the entire drive fitting in a 10.5-inch-high rack space (six rack units). In the mid-to-late 1980s, the similarly sized [[Fujitsu Eagle]], which used (coincidentally) 10.5-inch platters, was a popular product.

With increasing sales of microcomputers having built-in [[floppy disk|floppy-disk drives (FDDs)]], HDDs that would fit to the FDD mountings became desirable. Starting with the [[Shugart Associates|Shugart Associates SA1000]], HDD ''form factors'' initially followed those of 8-inch, 5¼-inch, and 3½-inch floppy disk drives. Although referred to by these nominal sizes, the actual sizes for those three drives respectively are 9.5", 5.75" and 4" wide. Because there were no smaller floppy disk drives, smaller HDD form factors such as 2½-inch drives (actually 2.75" wide) developed from product offerings or industry standards.

{{As of|2025}}, 2½-inch and 3½-inch hard disks are the most popular sizes.  By 2009, all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch form factors due to falling prices of [[flash memory]],<ref name="AutoMK-51" /><ref name="AutoMK-52" /> which has no moving parts. While nominal sizes are in inches, actual dimensions are specified in millimeters.