A word processor has become one of essential devices used in offices. Document files created by using a word processor are generally preserved on a floppy disk or floppy disks for references. Nevertheless, the files stored on a floppy disk are occasionally lost due to human errors, the life of the floppy disk and a failure of hardware devices, which comprise the word processor. This is called a floppy disk failure. One of the simplest methods for protection against such serious losses is to backup files on another floppy disk. Frequent backup operations would spend much time in themselves although they could reduce the loss at a floppy disk failure. On the contrary, occasional backup operations would make the loss at a floppy disk failure very large although they could save time in backup operations. These observations reveal the significance of determining an adequate backup timing of files. The present study proposes a 1/N backup policy for data floppy disks, which suggests to backup files when 1/N of the total memory of a floppy disk is consumed. The availability of the proposed backup policy is formulated as an objective function under efficiency basis. The existence of an optimal integer N* that maximizes the availability is then examined to clarify the conditions under which such an optimal integer exists. Numerical examples are also presented.
