Hard disk drives consist of one of more rigid disks (or platters) with magnetic heads arranged on a moving actuator arm to read and write data to the surfaces.
The 2.5-inch form factor is one of the two dominant types on the market, with the 3.5-inch form factor being the other. 2.5-inch hard disk drives were introduced in 1988 by PrarieTek and have become most common in laptops and other mobile devices, as well as game consoles such as the PlayStation 3 and Xbox 360.
They are normally 9.5mm high with a single platter, but taller drives with two or more platters have been produced, as well as smaller sizes (a 5mm version by Western Digital was introduced in 2013 for use in UltraBooks).
Whilst not strictly speaking removable media, 2.5-inch hard disk drives form the basis for many external hard disk drives such as those connected by USB, and docks are available to read hard disk drives without installing them.
Solid-state drives using flash memory are beginning to replace hard disk drives for uses where speed, power consumption and durability are more important considerations, such as in tablet computing.