A network user's biggest fear is a server hard disk failure. When any other piece of network server hardware fails, you lose access to the server while replacing the failed component. The only way to guard against the disaster of losing files on a server disk is to make sure that a backup copy of the files exists.
Many devices are available to assist in making backup copies of data.
The most commonly used kind of backup device for networks is a tape backup system. Tape backups offer several advantages. Tape cartridges are relatively inexpensive and can hold a great amount of data, anywhere from 100M to 10G. You can overwrite tapes, so you can use the same tape many times. Tape backups, however, have some disadvantages. Writing to a tape is a slow process.