A method of controlling a storage device attached to a host device, includes transferring data with respect to the host device in synchronism with a rise edge and a fall edge of a clock signal in first and second transfer modes. In response to a check function command, status data indicating that the storage device supports the second transfer mode is output through a data terminal. In response to a set function command, the storage device is set into the second transfer mode. The check function and the set function are switched in response to a first value and a second value of a mode bit in the check function command.