RAID control of multiple hard disk drives in a computer system includes performing a fault-tolerant data computing operation for a written data. The timing for performing the fault-tolerant data computing operation is determined by accessing a data stored in one of the hard disk drives, detecting a partial data length of a data stream having been transmitted from the hard disk drive to the computer system, issuing a triggering signal when the data length has reached a unitary length less than the total length of the data stream, and then performing the fault-tolerant data computing operation with the unitary length of data in response to the triggering signal