Abstract: A data eraser for erasing data recorded on a data recording medium, where data recorded on the recording medium can be erased in a short time period. Data produced in the CPU of the controller is stored in the memory as a random unit data; the unit data are repeatedly and continuously written in the memory to constitute record data for erasing purpose; the thus produced recording data for erasing are written on the hard disc to erase the data memorized in the hard disc. Since the time for producing random data, which normally takes time, is shortened, the time for proceeding as a whole can be shortened.

Abstract: A data back up apparatus which has a control portion 3, that is connected to a cable 23 being connected to a mother board of a computer system, a cable 24 being connected to a hard disk drive of the computer system, and a cable 5 being connected to a hard disk drive provided in the data back up apparatus, respectively; the control portion 3 always keeps the connection between the mother board and the hard disk drive provided in the data back up apparatus in off, and has a semiconductor switch 34 which switches the connection of the hard disk drive of the computer between the mother board and the hard disk drive provided in the data back up apparatus; the control portion 3 controls the data back up operation and the data restoring operation and the switching operation of the switch 34.