Hard Disk Drive

Most of us have hard disks in our computers now which are used as a secondary storage devices. As you know that secondary storage devices have non-volatile memory while Primary storage devices like D-RAM have volatile memory.

Ever wondered what lies inside the plastic case of your HDD and how it works. I will show you how..

The above figure is the internal structure of a typical hard disk drive. The hard disk doesn't contain a single disk but a bulk of them which look like CD's and are called Platters. The surface of each platter is logically divided into tracks which are inturn divided into sectors. The both sides of platter are covered with magnetic material. The information is magnetically stored on these materials. There will also be read-write heads used for reading or writing data. These are attached to a disk arm, that moves all the heads as a unit. All the tracks at one arm position form a cylinder.

Transfer time is the time needed for transferring of data between the hard drive and computer. It has two parts

1. The time needed to move the disk arm to the desired cylinder called seek time and
2. The time needed for the desired sector to rotate under the read-write head called rotational latency

Did you ever come across a term called head crash. Head crach occurs when the read-write head makes contact with the disk surface actually. The read-write head just flies on a thin cushion of air above the platter. Although the platters are covered with a thin protective layer, there may be times, when the head damages the magnetic surface. Head crash is irrepairable and the disk must be replaced.