A hard-disk drive is a magnetic storage device. Its data-storage principle is like that of a floppy-diskette drive. However, its storage medium is composed of many disks with each disk being made of hard plastic covered with a magnetizable material and with each disk side having a corresponding read/write head. As a consequence, a hard-disk drive is expensive. Hard plastic implies no wobbling and a read/write head can be placed so close to a disk’s surface. This implies the usage of weak magnetic fields and hence the feasibility of higher track density (high capacity). Also no wobbling implies fast rotating speed (short disk access time).

Typical hard-disk drives have rotating speeds ranging from 3600 to 9600 rotations per minute (rpm). Since the read/write heads are so close to the disks surfaces, hard-disks drives are very sensitive to dirt. That is partly why the drive motor, the read/write heads, and the storage disks are all enclosed in a sealed unit. Hard-disk drives are mostly fixed, but some are removable. However, because of their big sizes, removable ones are not as portable as floppy diskettes. Typical capacities are 20, 40, and 80 GB.