An external storage device

In an external storage, an I/O process is continued without any intervention of a user or a host system at failure of a controller. When a failure occurs in a controller (30), a host system 10 recognizes the failure of the controller (30). Before the failure is notified to the user and application to stop the job, the substitutive controller (40) reads the SCSI-ID possessed by an SCSI port (31) of the failed controller (30) from a shared memory (50), registers the SCSI-ID of the SCSI port (31) to the SCSI port (41) associated with the substitutive controller (40), and erases by a port address resetting facility 45 of the substitutive controller (40) the SCSI-ID possessed by an SCSI port (31) of the failed controller (30). Thanks to the provision, since the SCSI-ID specified at issuance of an I/O request is transferred between the controllers, the user or the host system need not alter the I/O request issuing route. Moreover, while the host system (10, 20) does not recognize the error, the transfer can be conducted.