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Secondary memory or storage is the non – volatile memory that stored externally from the computer. A secondary – storage medium is usually used for the storage of large amount of data or for permanent or long – time storage of data or programs. It is also used for storing backups.

Secondary storage media can be ranked according to the following:

(i)   Retrieval speed: This is the time it takes to locate and retrieve the stored data.

(ii)   Size / Storage capacity: This is the ability to store data. Large storage capacity is always desired

(iii) Cost per bit of capacity:  Low cost is preferred


(i)    Magnetic Disk: This is a Mylar or metallic platter on which electronic data are stored. The data are recorded as tiny invisible magnetic spots on its iron oxide coating. The access time for data stored on a magnetic disk is determined by the seek time i.e. the time required for positioning read/write head over the paper track and search time i.e. the time required or spinning the required data under the head. Once the data have been accessed, they are copied from the disk to the processor for processing. The transfer rate depends on the density of the stored data and rotational speed of the disk.

Magnetic Disk came in two categories which are:

(a)    Fixed disk or Hard disk: It is made from materials such as aluminium instead of Mylar.  The input/output device used to transfer data to from a hard disk called dish drive. Hard disk may be permanently installed in a drive or in form of removable disk.

Forms of Fixed disk or hard disk

1.      Larger permanent sealed metal disk (14 – inch) which is used in larger systems and minicomputer.

2.      Larger metal disk in removable cartridges (14 – inch) which is used in mini sized and larger systems.

3.      Rigid disks permanently housed in Winchester drives. It comes in 9, 8 and 5¼  inches size.

4.      3½ inch disks currently used in PC’s, servers and small ones in Notebooks and other device.


(b)   Floppy Disk: These disk are available in three sizes which are:

1.      8 inch portable floppy (flexible) disk.

2.      5¼ inch portable floppy disks.

These two diskettes are individually packaged in protective envelopes.

3.      Compact floppy disk measuring less than 4 inches in diameter.



This involves the use of laser beams i.e. highly concentrated beams of light. It comes in form of:

(a) Optical laser disk which could be Compact Disk Recordable (CD – R), Compact Disk Rewriteable (CD – RW) or Digital Versatile Disk (DVD) or flash  which the data stored in it can be read by CD – ROM drives, CD – RW drives, DVD – ROM drives or DVD – RW drives and Flash drive respectively.

(b)               DVD drive: A digital versatile Disk (DVD) is very similar to CD, but it has much larger data capacity. A dvd is made up of several layers of plastic totalling about 1.2 millimetres thick. DVD are also of same types as CD along with different formats as explained about CD


This tiny device makes it so easy to take your files with you, and even easier to use them when you reach your destination. The plug and play pen drives comes in capacity ranging from 16 mb to 1GB. They are faster and more reliable than floppy disks.

Comparison of Memory devices

The following table summarizes the characteristics of various kinds of data storage in the storage hierarchy

Storage Speed Capacity Relative Costs Permanent?
Registers Fastest Lowest Highest No
RAM Very Fast Low/moderate High No
Floppy Disk Very slow low Low Yes
Hard Disk Moderate Very High Very Low Yes
The teacher summarizes the lesson and allows student to ask questions to clear doubts.


1.       What is secondary memory

2.       Give three examples of secondary memory and expalin

Student are allowed to give corrections to the assessment given by the teacher, while the teacher support s them in order to guide them.


Make a research online about the differences between secondary and primary memory.


See also

Memory Unit

Central Processing Unit




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