Why do we need secondary storage device? about Hard disk. | web4study

Why do we need secondary storage device? about Hard disk.


Secondary storage devices are also called backup storage because it is used to store data. The volume of data on the permanent basis which can be partially transferred to the primary storage, when required for data processing. Afterwards, these devices are comparatively cheap and provide greater space to store the data /instructions are stored on secondary storage devices in the same binary codes as in primary storage.



The storage capacity of the primary storage of today’s computer is not sufficient. To store a large volume of data as a result additional memory called secondary storage is needed with most of the computer system.

These devices also provide the fast communication than 1/0 devices. The internal memory of a computer is a volatile memory. Therefore, we cannot save the data permanently. In that case, we require secondary storage device which provides the facility to store the data for future use.



The hard disk is a thin circular metal plate coated both sides with a magnetic material. A hard disk pack consists of a number of disks mounted on a central shaft which rotate at a speed of 2400 rpm or more. In a hard disk, information is stored on both the surfaces of each disk plate except the upper and the lower surfaces of the bottom plate which are not used.

Information is recorded on the track of the disk surfaces in the form of the invisible tiny magnetic spot. The presence of a magnetic spot represent’1 bit and its absence represents 0 bit.

The hard disk is potentially very high capacity storage devices typically in the range of 20 megabytes to 1 gigabyte. Data are recorded on the tracks of a spinning disk surface and read from the surface by one or more read/write heads. 

There are two basic types of the disk system.

1. Moving head. 

2. Fixed head



The moving head consists of one read/write head for each disk surface mounted on an axis and which can be moved in and out. In this system, each read/write head moves horizontally across the surface of the disk.

So that it is able to access each track individually. Information stored on the tracks which constitute a cylindrical shape through the disk pack is therefore accessed simultaneously. 



In the fixed head system the axis is non-movable. A large number of reading/write heads are distributed over the disk surfaces. One head for each track, as a result, no head movement is required and therefore information is accessed more quickly.


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