A hard disk is a rigid disk inside a computer that stores and provides relatively quick access to large amounts of data. It is the type of storage most often used with Windows. The system also supports removable media. The file system provides an abstraction of the physical characteristics of storage devices so that applications can simply write to and read from files. However, storage in an enterprise relies heavily on the concept of disks.
Disk Devices and Partitions
A hard disk consists of a set of stacked platters, each of which has data stored electromagnetically in concentric circles, or tracks. Each platter has two heads, one on each side of the platter that reads or writes data as the disk spins. A hard disk drive controls the positioning, reading, and writing of the hard disk. Note that the heads of all platters are positioned as a unit. The smallest addressable unit of a track is a sector. A disk sector is a wedge-shape piece of the disk. Each sector is numbered and a track sector is the area of intersection of a track and a sector. A cluster is a set of track sectors, ranging from 2 to 32 or more, depending on the formatting scheme in use.
A cylinder is defined as the set of tracks that appear in the same location on each platter or a set of matched tracks. For example, the following diagram shows a hard disk with four platters. Cylinder X consists of eight tracks (track X from each side of each platter).
A hard disk can contain one or more logical regions called partitions. Partitions are created when the user formats a hard disk as a basic disk. The creation of multiple partitions on a drive allows the appearance of having separate hard drives. For example, a system with one hard disk that has one partition contains a single volume, designated by the system as drive C. A system with a hard disk with two partitions typically contains drives C and D. Having multiple partitions on a hard disk can make it easier to manage the system, for example to organize files or to support multiple users. The first physical sector on a basic disk contains a data structure known as the Master Boot Record (MBR). The MBR contains the following: