Secondary Storage Devices

Secondary or Auxiliary storage are used to supplement the limited storage capacity of the primary storage section. these devices are On-line to the Processor. They accept data and programs from the processor, retain them and write them back as required. Both floppy disks and rigid disks are secondary storage for PCs. The rigid disks are generally sealed in their storage devices unlike floppies, which are removable. Floppies are Off-line. Once it is removed the data or instructions are inaccessible.

Some of the secondary storage devices are:

1. Magnetic Disks:  A disk drive can be used as an input/output media. It is a flat circular plate in aluminium, coated with ferric or chromium oxide. Known as Direct Access Storage Device (DASD), it need not be processed sequentially. So it is flexible and faster. Characters are recorded by magnetizing microscopic areas on the disks surface. It is mounted on a spindle that causes it to rotate. A Read/Write head that is positioned by the disk drive moves back and forth across the disks' radius retrieving or storing data. Data may be recorded on both sides of a disk.
2. Disk Packs:  Many hard disks attach several disks to one spindle and the disks rotate together. It floats on an arm without touching the surface. This is known as Disk Pack. The R/W head is mounted on access arms allowing it to move from one track to another. the total collection of tracks available on one movement is known as Cylinder. The speed is instant, storage capacity is enormous. The disk diameter is 14".
3. Hard Disks or Fixed Disks:  HDs may be fixed on their drives or may be removable. They are usually about 14" diameter. Smaller ones are used in micro-computers. One such system is known as winchester or mini-winchester uses 8" or 6 1/4" platters. These are sealed in their drives.
4. Flexible Disks, Diskettes:  These are called floppy disks or floppies. It is a thin plastic sheet base and are used in micro and mini-computers. It sizes are 3 1/2", 5 1/4" and 8".
5. Magnetic Tape:  It consists of a long strip of plastic similar to video/audio tapes, coated with an iron oxide compound that is magnetized. it is wound on a 10 1/2" reel for use in Main Frame and Mini Computers. Data are recorded and read using a tape drive. they are organized into records called Blocking Factor. A group of such records is called Block. It is a sequential media and is slower than floppy or hard disks.
6. Mass Storage Systems:  Industry's need for machine-readable storage is increasing. Thus Data Catridge system was developed. It can store data in a series of 50MB catridges. A catridge is loaded into the R/W head and after processing it is stored away. This system is however slow.

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