A portable computer including a CPU, a detachable hard disk pack which is connectable to a system bus of the portable computer, and a hard disk pack replacement detector which detects, in a power-Off state, a detachment of the disk pack and sets a signal in a status register indicating the detachment of the disk pack. In response to a power-ON state, the CPU reads the status register to determine whether the disk pack had been detached during the power-OFF state and, if so, disables a resume flag so that operation does not resume at the operating state of the portable computer which existed immediately preceding the power-OFF state.