MAGNETIC DISK

A magnetic disk is a Mylar-or metallic platter on which electronic data are stored. Unlike tapes, data on magnetic disks can also be read randomly. The data are recorded as tiny invisible magnetic spots on its iron-oxide coating. Read/write heads are tiny electromagnets that can read, write, or erase the polarized spots that represent data on magnetic media. These heads are fastened to an arm in a disk storage device so that they can be moved quickly and directly to any disk location| to store or retrieve data.

The access time for data stored on a magnetic disk is determined by two factors:

1.  The seek time i.e. the time required for positioning read/write head over the proper track.

2.  The search (latency) time (Rotational Delay)i.e. the time required for spinning the required data under the head.

Once the data have been accessed, they are copied from the disk to the processor for processing.
The transfer rate depends on the density of the stored data and the rotational speed of the disk.

Magnetic disks come in various sizes. They can be portable or permanently mounted in their
storage devices, disk drives. They can be made of rigid metal (Hard Disks) or flexible plastic (Floppy Diskettes). They can be divided into two broad categories:

1. Fixed Disks or Hard Disks

2. Floppy Diskettes

On all disks, there are a number of invisible concentric circles called tracks on which the data are
stored. These tracks begin at the outer edge of the disk and continue towards the center of the disk
without touching it. Every track on a disk has a designated number. Each track is divided into
portions called sectors.

Storage capacity of a disk depends on:

disk surface - The more the disk surface, the greater is Its storage capacity.

bits per inch of track - Storage capacity depends on the number of bits that can be stored on an inch of track. Both are directly proportional.

tracks per inch of surface - The greater the number of tracks that can be placed on an Inch
of surface, the greater is the storage capacity of the disk.

Advantages of Magnetic disk:

• Data records can be stored for both sequential and direct accessing.
•  High speed of access and update as no sorting of transactions.
• On-line disk records of several related files can be updated simultaneously by a single input transaction.

Disadvantage of magnetic disk:

• Risk of loss of data: Records may be lost in case of writing of new records or drive failure. Therefore, special backup procedures are required for disk records protection. This is not so in the case of magnetic tape records.
• Data insecurity: it is easier to maintain the security of magnetic tape files than that of magnetic disk files.
• Expensive: Magnetic disks are expensive compared to magnetic tapes.

Floppy Disk

Hard Disk