Data is updated while making sure that one logically meaningful data group is set as the unit of data update. A host computer requests a first disk controller a data input/output request to/from a logical volume of a first disk drive. When the request is an update request, requested update data is stored in the first logical volume, by a first control. The update request is sent to a second disk controller and is temporarily accumulated there, when information indicating a group to which the update data belongs is attached to the update request. When the first disk controller finishes executing every data input/output request that is designated by information indicating to which group the update data belongs, the accumulated update data for those belonging to the same group to which group update data belongs is retrieved and stored in a second logical volume.