The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops.
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Performance is specified by the time required to move the heads to a track or cylinder (average access time) plus the time it takes for the desired sector to move under the head (average latency, which is a function of the physical rotational speed in revolutions per minute), and finally the speed at which the data is transmitted (data rate). Typically, some of an HDD's capacity is unavailable to the user because it is used by the file system and the computer operating system, and possibly inbuilt redundancy for error correction and recovery. Capacity is specified in unit prefixes corresponding to powers of 1000: a 1-terabyte (TB) drive has a capacity of 1,000 gigabytes (GB where 1 gigabyte = 1 billion bytes). The primary characteristics of an HDD are its capacity and performance. As of 2015, HDD production (exabytes per year) and areal density are growing, although unit shipments are declining. More than 200 companies have produced HDD units, though most current units are manufactured by Seagate, Toshiba and Western Digital. Continuously improved, HDDs have maintained this position into the modern era of servers and personal computers. Introduced by IBM in 1956, HDDs became the dominant secondary storage device for general-purpose computers by the early 1960s. HDDs are a type of non-volatile memory, retaining stored data even when powered off. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored or retrieved in any order and not only sequentially.
The platters are paired with magnetic heads arranged on a moving actuator arm, which read and write data to the platter surfaces. A hard disk drive (HDD), hard disk, hard drive or fixed disk is a data storage device used for storing and retrieving digital information using one or more rigid ("hard") rapidly rotating disks (platters) coated with magnetic material.
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