Hard disk is a secondary storage device. It is a disk pack consisting of several metal platters coated with a magnetic material, rotating at high speed (7200 revolutions per second). Each surface has a read/write head mounted on an arm. The heads can only move in one radial direction. Both sides of a platter are used. Each surface is divided into concentric circles or tracks, typically 1000 tracks per platter.

Each track is divided into about 100 sectors, separated by gaps. To access data in a specific location, the arm has to be positioned over the proper track, this takes about 10-20 ms (1 ms = 1 millisecond = 0.001 s). The head waits until the desired sector is under it; this takes about 6 ms. Once the sector is found, the data can be read at a rate of 4 Mb/s.
Hard Disk Components:
A number of disks on which data is stored (the more disks, the greater the drive’s capacity).

• A motor to rotate the disks
• Ahead to read and write data (The heads “fly” over the surface of a disk supported by a few molecules of air)
• An actuator to move the heads over the disks

Each disk is segmented radially into tracks. Tracks are about 300 millionths of an inch apart. Each track is divided into sectors. A sector is the smallest addressable unit on a direct access storage device.

A cylinder is made up of all the tracks that are at the same location of each platter in the device.
The storage capacity of the hard disk ranging from several GigaBytes (250GB) to several Terra Bytes (2 TB). There are internal hard disk and external hard disks.