PURPOSE: To guarantee the safety of data and to improve the performance by putting ≥2 disk drives in parallel operation and doubling files with the same performance with one drive.

CONSTITUTION: When a file store (write) instruction is sent to a microprocessor(MPU) through a communication controller(CCU) 3, the MPU writes a file on hard disk drives(HDD) 5 and 6 from a main storage device(MMU) 2 through a system bus 4. In this disk writing operation, the controllers of the HDDs 5 and 6 interpret the write instruction in the same way respectively to perform the same operation, so that the same performance with one drive is obtained. When a load (read) instruction is sent, on the other hand, only the HDD #0 5 operates normally to load the read data onto the MMU 2 and the read data is loaded again at this time. Thus, the same HDDs are put in parallel operation for writing and switched and operated for reading to double files with the same performance. Consequently, the safety of data is guaranteed and the performance is improved.