Computer system and data control method

In failover processing, a CPU restores data stored in a first volume to a second volume of a storage system, associates a unique ID of the first volume with the second volume, and stores the unique ID associated therewith in a memory. After the failover processing is completed, the CPU manages an update difference management bitmap indicating an updated content with respect to data stored in the second volume. The CPU transmits, in failback processing, update data updated after the failover processing among the data stored in the second volume to the first volume identified by the unique ID associated with the second volume based on the update difference management bitmap.