Secondary Storage Devices

It is important to know the difference between secondary storage and a computer's main memory. Secondary storage is also called auxiliary storage and is used to store data and programs when they are not being processed. Secondary storage is more permanent than main memory, as data and programs are retained when the power is turned off. The needs of secondary storage can vary greatly between users. A personal computer might only require 20,000 bytes of secondary storage but large companies, such as banks, may require secondary storage devices that can store billions of characters. Because of such a variety of needs, a variety of storage devices are available. The two most common types of secondary storage are magnetic tapes and magnetic disks.

Magnetic Tape Storage

Magnetic tape is a one-half inch or one-quarter inch ribbon of plastic material on which data is recorded. The tape drive is an input/output device that reads, writes and erases data on tapes. Magnetic tapes are erasable, reusable and durable. They are made to store large quantities of data inexpensively and therefore are often used for backup. Magnetic tape is not suitable for data files that are revised or updated often because it stores data sequentially.

Magnetic Disk Storage

Magnetic disks are the most widely used storage medium for computers. A magnetic disk offers high storage capacity, reliability, and the capacity to directly access stored data. Magnetic disks hold more data in a small place and attain faster data access speeds. Types of magnetic disks include diskettes, hard disks, and removable disk cartridges.

Diskettes

The diskette was introduced in the early 1970s by IBM as a new type of secondary storage. Originally they were eight inches in diameter and were thin and flexible which gave them the name floppy disks, or floppies. Diskettes are used as the principle medium of secondary storage for personal computers. They are available in two different sizes: 3 1/2 inch and 5 1/4 inch.

DISKETTE STORAGE CAPACITY

• Before you can store data on your diskette, it must be formatted (G). The amount of data you can store on a diskette depends on the recording density and the number of tracks on the diskette. The recording density is the number of bits (G) that can be recorded on one inch of track on the diskette, or bits per inch (bpi). The second factor that influences the amount of data stored on a diskette is the number of tracks on which the data can be stored or tracks per inch (tpi). Commonly used diskettes are referred to as either double-density or high-density (single-density diskettes are no longer used). Double-density diskettes (DD) can store 360K for a 5 1/4 inch diskette and 720K for a 3 1/2 inch diskette. High-density diskettes (HD) can store 1.2 megabytes (G) on a 5 1/4 inch diskette and 1.44 megabytes on a 3 1/2 inch diskette.


CARE OF DISKETTES

• You should keep diskettes away from heat, cold, magnetic fields (including telephones) and contaminated environments such as dust, smoke, or salt air. Also keep them away from food and do not touch the disk surface.

Hard Disks

Hard disks provide larger and faster secondary storage capabilities than diskettes. Usually hard disks are permanently mounted inside the computer and are not removable like diskettes. On minicomputers (G) and mainframes (G), hard disks are often called fixed disks. They are also called direct-access storage devices (DASD). Most personal computers have two to four disk drives. The input/output device that transfers data to and from the hard disk is the hard disk drive.

HARD DISK STORAGE CAPACITY

• Like diskettes, hard disks must be formatted before they can store information. The storage capacity for hard drives is measured in megabytes. Common sizes for personal computers range from 100MB to 500MB of storage. Each 10MB of storage is equivalent to approximately 5,000 printed pages (with approxiamtely 2,000 characters per page).

Disk Cartridges

Removable disk cartridges are another form of disk storage for personal computers. They offer the storage and fast access of hard disks and the portability of diskettes. They are often used when security is an issue since, when you are done using the computer, the disk cartridge can be removed and locked up leaving no data on the computer.

Backup

Backup means creating a copy of important programs and data. To backup diskettes, copy the data from one to the other. Diskettes are frequently used to backup important files on hard drives of personal computers.

Data Recovery

There does exist certain software programs that, under special circumstances, can recover data and programs that have been "lost." Often your operating system (G) will be able to do this, or applications such as Norton Utilities.