Storage system control method, storage system, information processing system, managing computer and program

When shifting the condition of a pair of a main volume and a sub-volume from a split condition to a pair condition according to a request from a user, an information processing unit which can access the sub-volume is inquired whether it mounts the sub-volume or not. As a result, if a managing computer receives a notice from the information processing unit that it mounts the sub-volume, the managing computer displays a warning on its display. Thereby, data on the sub-volume can be prevented from being erased by an operation mistake of the user.