A method and system are disclosed to prevent significant degradation of a data transfer rate in write cache transfer, thereby keeping the minimum data transfer rate high. The initial value Ti of the virtual buffer full capacity T(tx) is set to F/N (where F is a data capacity of the buffer, and N>1) and it is determined if the amount of write cached data S(tx) reaches the virtual buffer full capacity T(tx) so that write cache transfer is enabled during an S(tx)