A disk array system of the type that each controller has an independent and dedicated cache. The disk array system can change control of a desired volume between desired controllers without suspending the system. When volumes are taken over between controllers, a switch-source controller de-stages data of a subject volume on the data cache to a storage subject disk to maintain the disk content reflection (coherency). Even if each controller has an independent and dedicated cache, a desired volume can be taken over between desired controllers without suspending the system. Each controller has a configuration manager which stores the controller number of a switch-destination controller to allow automatic volume take-over and automatic control by the original controller.