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 3.5-inch form factor hard disk drive is one of the two dominant types on the market, with the other being 2.5-inch. The 3.5-inch form factor hard disk drive was introduced by Rodime, a Scottish company, in 1983. Rodime already built hard disk drives using the 5.25-inch form factor, but introduced the smaller size that utilised the form factor already introduced for 3.5-inch floppy disk drives.
The first 3.5-inch hard disk drive had a capacity of 10 MB. After its introduction, many competitors introduced hard disk drives using the 3.5-inch form factor, and Rodime sued many other disk manufacturers for infringement of its patents.
Whilst called 3.5-inch, the drives occupy a space 4-inches wide, and were initially 1.6-inches high (the same as the then current ‘half-height’ 3.5-inch floppy disk drives), but the most popular size today is the 1-inch high ‘slimline’ or ‘low-profile’ version.
As of 2014, the largest capacity 3.5-inch hard disk drive is 10 TB, equal to 1,000,000 times the capacity of the first disk.
Whilst not strictly speaking removable media, 3.5-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 many 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.