A storage system comprises a plurality of control modules having a plurality of cache memories respectively. One or more dirty data elements out of a plurality of dirty data elements stored in a first cache memory in a first control module are copied to a second cache memory in a second control module. The one or more dirty data elements stored in the second cache memory are backed up to a non-volatile storage resource. The dirty data elements backed up from the first cache memory to the non-volatile storage resource are dirty data elements other than the one or more dirty data elements of which copying has completed, out of the plurality of dirty data elements.