Abstract: In a portable computer having an external-memory loading/unloading mechanism and a resume function, presence or absence of change of the loaded condition of a hard disk pack is determined while the portable computer is in a resume mode when the portable computer is powered. If the condition has changed, a resume error is displayed, instructing an operator to continue operation, ignoring the resume error, or to perform the operation from the beginning. If the operator continues the operation, a drive designation of the computer is changed. When the power of the portable computer is cut off, the ID data identifying a hard disk pack is written into the pack and also a backup RAM. When the portable computer is powered, the ID data is read from the pack and compared with the ID data stored in the backup RAM. If the ID data items compared are identical, the resume function is performed, thereby preventing the hanging up of the system.