PURPOSE: To automatically record a charged data with no intervention of a software, by preparing two external storage devices of the same type and storing an original data in one of the two storage devices and only the changed data in the other one respectively.

CONSTITUTION: A magnetic disk device 4 stores an original data, and another magnetic disk device 5 stores the changed data respectively. These disks 4 and 5 are controlled by a controller 2. An address map memory 3 shows the state of the memory space on the disk device. A processor 1 produces an input/output instruction to the controller 2 when an access is given to the disks 4 and 5 respectively. The controller 2 refers to the memory 3 to decide which disk should be accessed.