A database update method for a computer system having a host computer and a storage sub-system where a database is maintained is disclosed. The storage sub-system includes first and second storage units containing duplicated data. The host computer accesses the first storage unit for data reading and the second storage unit for data writing. If incomplete processing to read is in process when an access request for data writing occurs, the host computer commands the stop of volume duplication of the first and second storage units and issues a write request. In response to this command, data is stored into only the second storage unit. Upon the completion of both read and write processing, the host computer commands the restart of volume duplication of the first and second control units to reflect the data stored into the second storage unit in the first storage unit.