to the host controller or the host computer which are fitted directly into the slots of the system bus of the host controller or the host computer numeral 15 designates an lsi chip in which is integrated the disc interface control circuit 7 of fig4 numeral 16 designates an lsi chip in which is integrated the host interface circuit 3 of fig4 numeral 17 designates an lsi chip containing the microprocessor of fig4 numeral 18 designates an lsi chip containing the buffer memory 6 of fig4 these lsi chips are individually mounted directly on the printed circuit board 10 moreover those small sized disc units lsi chips and fitting connectors are interconnected as necessary over the printed circuit board 10 specifically since the disc array device 1 of fig4 has all of its functions mounted on the single printed circuit board 10 it can be mounted directly in the system slot of the host controller or the host computer so that it can be used as an internal file connected directly to the bus as a result no individual cables are required for connecting the small sized disc units to each other and connecting the small sized disc units to the individual lsi chips of the array control unit the small sized disc units and the array control unit can be connected thorugh a short bus and can be connected to the host controller or the host computer via a fast serial interface so that the data transfer can be accelerated moreover since the small sized disc units are mounted directly on the board no support mechanism is required for the small sized disc units so that the cost can be lowered thus the.