A hard disk is part of a unit, often called a “disk drive,” “hard drive,” or “hard disk drive (HDD),” that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today’s computers typically come with a hard disk that contains several billion bytes (gigabytes) of storage.
A Hard disk can also be defined as:
- a rigid (“hard”) non-removable magnetic disk with a large data storage capacity.
- 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.
- A magnetic disk on which you can store computer data. The term hard is used to distinguish it from a soft, or floppy disk. Hard disks hold more data and are faster than floppy disks.
A hard disk is really a set of stacked “disks,” each of which, like phonograph records, has data recorded electromagnetically in concentric circles or “tracks” on the disk. A “head” (something like a phonograph arm but in a relatively fixed position) records (writes) or reads the information on the tracks. Two heads, one on each side of a disk, read or write the data as the disk spins. Each read or write operation requires that data be located, which is an operation called a “seek.” (Data already in a disk cache, however, will be located more quickly.)
A hard disk/drive unit comes with a set rotation speed varying from 4500 to 7200 rpm. Disk access time is measured in milliseconds. Although the physical location can be identified with cylinder, track, and sector locations, these are actually mapped to a logical block address (LBA) that works with the larger address range on today’s hard disks.