PURPOSE: To limit a partial fault of a storage medium by securing a data by a logically divided unit, in case of processing the data divided logically into plural pieces between two storage mediums.

CONSTITUTION: At the time of loading, the contents of a control information sector 8 are inputted to a control information area 6 of a semiconductor memory 5 from a magnetic disk 7, subsequently, a disk valid flag corresponding to "0" number of a logical volume existing in the control information area 6 is checked, and if this flag is on, a data in the magnetic disk part 7 is transferred to a volatile semiconductor memory 5. At the time point when loading of one volume portion has been completed, a memory valid flag is turned on and the disk valid flag is turned on. In case when there is a fault in the magnetic disk and loading is impossible, an effective data does not exist but the semiconductor memory can be used, therefore, the next volume processing is executed. Also, when there is a fault in the memory 5 and loading is impossible, the next processing is executed in a state that the disk valid flag in remaining on.