A few special cases about tape handling warrant more detailed
description. These special cases are discussed below.
Many complexities surround the use of tar on tape drives. Since
the creation and manipulation of archives located on magnetic tape was
the original purpose of tar, it contains many features making
such manipulation easier.
Archives are usually written on dismountable media—tape cartridges,
mag tapes, or floppy disks.
The amount of data a tape or disk holds depends not only on its size,
but also on how it is formatted. A 2400 foot long reel of mag tape
holds 40 megabytes of data when formatted at 1600 bits per inch. The
physically smaller EXABYTE tape cartridge holds 2.3 gigabytes.
Magnetic media are re-usable—once the archive on a tape is no longer
needed, the archive can be erased and the tape or disk used over.
Media quality does deteriorate with use, however. Most tapes or disks
should be discarded when they begin to produce data errors. EXABYTE
tape cartridges should be discarded when they generate an error
count (number of non-usable bits) of more than 10k.
Magnetic media are written and erased using magnetic fields, and
should be protected from such fields to avoid damage to stored data.
Sticking a floppy disk to a filing cabinet using a magnet is probably
not a good idea.