A removable hard disk drive is detachable from a computer apparatus, and includes a storage medium for storing data from the computer apparatus and a communication interface with the computer apparatus. Upon reception of eject instruction issued by the computer apparatus, the removable hard disk drive writes contents cashed in a cache memory into a disk, and shifts to an ejectable state. Upon the completion of the shift to the ejectable state, the removable hard disk drive externally outputs an eject signal. A microprocessor which controls an eject mechanism executes removable hard disk cartridge eject operation in response to the eject signal.