Magnetic Disk

The magnetic disk is the most widely used storage media. A magnetic disk is a thin, circular metal plate/platter coated with magnetic material. Information can be recorded on or read from the magnetic surface. A disk must be formatted before it can be used. The formatting process prepares the disk so that it um store data.

The formatting process creates tracks and sectors on the disk. Each track on a disk is divided into sectors. Each sector holds a specific number of characters. A sector is the smallest physical storage unit on a disk. Its size is normally 512 bytes. A cylinder consists of all tracks of the same number. Every cylinder contains the same number of sectors. For example, track 0 of side 1 and track 0 of side 2 is called cylinder 0.

The magnetic disk is a random access storage media. It means that any part of the disk is directly accessible Following are the three types of magnetic disks:

1. Floppy Disk
2. Hard Disk
3. Zip disk

 

Floppy Disk or Diskette

Floppy disk is also called diskette. It consists of a thin plastic disk coated with magnetic material. This disk is enclosed in a plastic jacket. It was introduced by IBM in the early 1970s. It is a portable storage medium and can be removed from one computer and inserted into another computer easily.

Floppy disks can only store a small amount of data. The data access speed of the floppy disk is slower than the hard disk. It is an inexpensive storage media.

The standard size of the floppy disk is 3 ½  inch. The capacity of floppy disks is 1.44MB. The circular piece of plastic on 3 ½  inch diskette is enclosed in a shell. A piece of metal covers the reading and writing area. It is called the shutter. When the disk is inserted into a disk drive, the shutter opens to expose the surface of the disk.

The data stored on a floppy disk can be read with a floppy disk drive. The mad/write head m the floppy disk drive is used to read and write data on the floppy disk. Floppy disk drives are downward compatible. It means they can use earlier media.

The storage capacity of a diskette depends on the following two factors:

1. Density: It is the number of bits that can be stored in one inch. It is measured as bits per inch (bpi). A floppy disk with higher density can store more data.
2. Number of Tracks: The number of tracks on a diskette is measured as tracks per inch (TPI). A floppy disk with more tracks can store more data.

 

Hard Disk

The hard disk is a type of magnetic disk. It is also called a fixed disk because it is fixed in the system unit. A hard disk consists of several circular disks called platters. The platters are used to store data. A platter in a hard disk is coated with magnetic material.

The hard disk provides a large storage capacity. It is much faster than floppy disks. It is the primary media for storing data and programs. The capacity of computer hard disk is from 2 to 80 GB and more.

 

Hard Disk Performance

The following factors affect the performance of the hard disk:

1. Seek time or positioning performance: How quickly the mad/write head positions itself and begins transferring information. It is measured in milliseconds (ms).
2. Spindle speed or transfer performance: How quickly the drive transfers data. It is measured in rotations per minute (RPM).
3. Latency: The time it takes for the spinning platter to bring the desired data to the read/ write head. It is measured in milliseconds.

 

Disk Cache

Disk cache is used to improve hard disk performance. It is a type of RAM that stores program instructions and data the user is working with. When the CPU needs information, it first looks in the disk cache. If it does not find the information it needs, it retrieves the information from the hard disk. As RAM is much faster than the disk, disk cache dramatically improves hard disk performance. On Macintosh computers, the disk cache is part of the main memory (RAM). On PCs, the disk cache is part of the hard disk.

 

Hard Disk Controller

A chip used to control the transfer of data, instructions, and information from disk to other units is called the disk controller. It is also known as an interface. A hard disk controller (HDC) is an interface for hard disk. It may be a part of a hard disk or separate card in the system unit. Two types of hard disk controllers for personal computers are as follows:

1. EIDE: EIDE stands for Enhanced Integrated Drive Electronics. It supports up to tour hard disk of 137 GB capacity each. The data transfer rate of EIDE is 66MB per second.
2. SCSI: SCSI stands for Small computer system interface. It supports multiple disk drives, CD—ROM drives, CD-RW drives, DVD-RM drives, tape drives, printers, scanners, network interface cards, etc. It is faster than EIDE. Its data transfer rate is 160MB per second.

 

Removable Hard Disks

Some hard disks are removable and can be inserted into hard disk drives like floppy disks. The removable hard disk is also called disk cartridge. It is a plastic or metal case that contains the hard disk. Two popular removable hard disks are jaz® disk and Peerless™ disk.

 

RAID

RAID stands for the redundant army of independent disks. It is a group of two or more hard disks that are integrated together. It is a more reliable but expensive system. RAID system duplicates data for more reliability in different ways.

The simplest storage design is called level 1 or mirroring. This design has one backup disk for each disk. If a disk fails, the data is available on another backup disk.

Another technique is called sniping. It splits data, instructions, and information on multiple disks. It does not duplicate data but increases data access time.

 

Difference between Diskette and Hard Disk

The difference between diskette and the hard disk is as follows:

 

Diskette

1. The diskette contains a single flat piece of plastic-coated with iron oxide.
2. It is small and portable.  
3. It is less expensive than a hard disk.
4. It can store a small amount of data.
5. Its data access snail is slower than a hard disk.
6. Data stored on the diskette is less safe than a hard disk.
7. It can be damaged easily due to dust and heat.

 

Hard Disk

1. The hard disk contains one or more metal plats coated with iron oxide.
2. It is usually fixed and nor portable.
3. It is more expensive than a diskette.
4. It can store large amounts of data.
5. Its data access speed is faster than a diskette.
6. Data stored on a hard disk is safer than a diskette.
7. It cannot be damaged easily due to dust and heart.

 

Zip Disk

Zip disk is a portable disk. It has more storage capacity than a floppy disk. Its storage capacity is up to 100 MB. Zip disk can be read/written by using a Zip drive. It is used for taking the backup of large data.

 

Super Disk

Super disk is produced by Imation. Its capacity is 120MB. The super disk drive can also read the standard 1.44 MB floppy disk. Zip drive cannot read standard 1.44 MB floppy disk.

 

HiFD Disk

HiFD disk is produced by Sony Corporation. Its capacity is 200MB. HiFD disk drive can also read the standard 1.44 MB floppy disk.

 

• Storage Devices