been duplexed a user specified port information table 640 shown in fig8 is stored in the management information unit 600 in the cache memory 500 this table contains a port usage value specified by the user via the personal computer workstation pc ws 3000 more specifically the user specifies a value indicating that the port is for backup processing only a value indicating that the port is for online processing only or a value indicating the port is for both online processing and backup processing when a backup request is received the disk array controller 300 references this information searches for a backup port and issues the request to the port controller of the port a copy backup timing instruction information table 650 shown in fig9 is stored in the management information unit 600 in the cache memory 500 this table contains information indicating when to execute copy processing for duplexing and subsequent backup processing specified via the personal computer workstation pc ws 3000 usually the pc ws is connected to the controller via a lan a time supervision instruction information table 660 shown in fig1 is stored in the management information unit 600 in the cache memory 500 when the copy backup timing instruction information table 650 shown in fig9 indicates a time the time information is set in the time supervision instruction information table 660 fig1 shows a priority information table 670 this table contains a value indicating to what processing online processing or backup processing the disk array controller assigns priority the user specifies this value in advance from the personal computer workstation pc ws 3000 the disk array controller 300 references this information when it starts backup processing if backup processing has priority the controller transfers a predetermined amount of data then the controller checks if.