External memory is a type of data storage device for a computer that is kept outside of the computer in its own casing. The hard disk, floppy disk, USB flash dish, optical disk (CD ( Compact Disk ), DVD ( Digital Versatile Disc )) etc are example of external storage device.
External memory which is sometimes called backing store or secondary memory, allows the permanent storage of large quantities of data.

 Some method of magnetic recording on magnetic disks or tapes is most commonly used.

More recently optical methods which rely upon marks etched by a laser beam on the surface of a disc (CD-ROM) have become popular, although they remain more expensive than magnetic media.
The capacity of external memory is high, usually measured in hundreds of megabytes or even in 
gigabytes (thousand million bytes) at present.

External memory has the important property that the information stored is not lost when the computer is switched off.