Magnetic Disks

Disadvantages of Magnetic Tapes

* Due to the sequential nature of storage and retrieval of data from the, they cannot be used to store data that need to be accessed randomly

* They have to be stored in a dust free environment because specks of dust on the tapes can cause tape­

reading errors

* They must be stored in locations with precise temperature and humidity levels

* Magnetic tapes can get easily warped and this will result in loss of data.

* They have to be correctly labeled else useful data stored on a particular tape can be easily erased by in error.

disk in form of tiny invisible magnetized and non- magnetized spots on the coated surface of the disk.

Magnetic disks are usually stored in a specially designed envelope or cartridge or several of them can be stacked together in a sealed, contamination-free container. Types of magnetic disks include computer hard disks and floppy disks.

* Computer Hard Disk

This is a round. Hat piece of inflexible metal, (usually aluminum), coated with magnetic oxide and usually fixed inside the cabinet of the computer's CPU (Central Processing Unit). Hard disks come in different sizes, usually from 1 inch in diameter to 14 inches in diameter. It is the commonest form of computer storage used today, in fact, the major and usually the largest storage device located in the computer system. It is stores all the programs that are used to operate the computer, (except the computer start-up programs that are burned into the computer ROM during manufacture). The hard drive also stores all the computer's data files and this include documents of all kinds, graphics, audio and video files, etc. and organizes all the files into folders and directories.

It is usually (but not always) permanently mounted inside the computer. When mounted in the computer, it is called an internal hard disk. Internal hard disks are usually bigger, faster, less portable and less expensive than external hard disks. The hard disk rotates while

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recording data and the rotation speed is measured in units of revolutions per minute (rpm). The usual speed of a hard disk is 3600 revolutions per second. Due to its large storage capacity, computers usually have multiple drives (locations) for storing tiles. The hard drive is generally the “C" drive. The other drives are also locations for external storage of tiles. Drives are generally labeled sequentially, i.e. A, C, D, E, F, G, etc. the “B” drive is no longer used. The “A” drive is used to save data from a floppy disk.

An Internal Hard Disk

Hard disks are of 3 types, i.e. Zip Disks, Disk Packs and Winchester Disk.

i. Zip Disks

These are hard disk that use a single plate encased in a plastic cartridge. They can be portable or fixed as part of the computer component. A portable zip disk can be carried to the computer system, connected to it for the

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duration of use and when work is done, disconnected and kept in a safe place.

ii. Disk Pack

These consist of multiple hard disk plates mounted on a central shaft. The disk pack drive has a separate read and write head for each usable disk surface (the upper surface of the topmost disk and the lower surface of the bottom disk is not used). Disk packs have to be mounted on the disk drive before they can be used, and can be removed and kept in a safe place when not in use.

iii. Winchester Disk

Winchester disk also use multiple (two or more) hard disk plates mounted on a central shaft. However, the disk plates and the disk drive are sealed together in a contamination-free container and cannot be separated from each other. Another difference in Winchester disk is that it has a larger storage capacity than disk pack for the same number of disks. This is because all the surfaces of the Winchester disks are used for data recording, resulting in greater data density.

* Floppy Disk

These are circular pieces of flexible plastic coated with magnetic oxide. It is also known as a diskette. They are usually protected by either a plastic or vinyl jacket to protect them from contamination of the sensitive data stored in them. They are used to store data and transfer them from one computer to the other. Floppies are called high density disks because they can hold large amounts of data. There are 2 types of floppy disks:

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a) The 3 Vi inch floppy i.e. its diameter is 3 Vi inches, and it is enclosed in a hard, plastic jacket

b) The 5 Vi inch floppy i.e. its diameter is 5 Vi inches and it is enclosed in a flexible vinyl jacket.

A Floppy Disk

When using a floppy disk, certain rules must be observed;

i. They should be kept from magnetized items

ii. They should not under any circumstances be bent or folded.

iii. The surface of the floppy disk must not be touched iv. Floppies should not be stacked one on top of the other v. They should be kept from heat and moisture (or humid

atmosphere)

vi. They should be kept in cases to protect them from dust vii. Avoid erasing and inputting data on floppies as much as

possible.

Advantages of Magnetic Disks

* They are more suitable for storing a wider range of applications than magnetic tapes. This is because

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they support direct and random access o f data unlike magnetic tapes which only support sequential access of data

* The random access characteristic peculiar to them allow them to be used simultaneously by multiple users as a shared device. A magnetic disk is not suitable for such use because of its sequential access character

* They are suitable for both online and offline storage of data i.e. both when the computer is on and when it is off

* Except for a Winchester diask. the storage capacity of a magnetic disk is virtually unlimited since as many disks as are necessary can be used for storing large data sets

* The cost per bit o f storage is very low because their low cost and high data recording density.

* They can be used and erased many times

* Floppy disks and Zip disks are easy to handle and store. This is because they are compact and light in weight. This also enables very large amount of data to be stored in a small storage space and makes them portable from place to place.

* They are less vulnerable to contamination due to careless handling or unfavorable temperature or humidity than magnetic tapes.

Disadvantages of Magnetic Disks

* They are less efficient to be used for sequential processing o f data

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* It is more difficult to maintain security of information stored in a magnetic disk.

* A disk crash or drive failure often results in loss of the entire data stored in them. This is especially true of Winchester Disks. Materials that can cause a head crash include human hair, fingerprints, dust particles and smoke particles.

* Some of them are not easily portable e.g. Disk Pack and Winchester Disk

* They are usually more expensive than the magnetic tape

* Zip Disks, Floppy Disks and Disk Packs must be properly labeled to prevent accidental erasure of important data.