Backup Storage:

Back up (backing) storage, also termed external storage, is used to hold programs and data which are read into internal storage when required. The most common form now, as for the last thirty years, is magnetic storage media, either tapes or disks. With both of these, data is recorded onto a plastic surface, coated with a varnish containing an oxide which can be magnetised in one or two directions; each magnetisation representing one bit of data. Data is written or read with the help of read/write heads, small electromagnets, close to the surface.