A computer system, storage system, and device control method which keep and operate long-term data stored in a disk device using its characteristics for a long time and at low cost, in a storage system or in a storage system using an external storage connection method. A computer system comprises a host computer, a storage system, and a management server, which are connected to one another via a network. The storage system receives a request to detach a logical device, specifies a logical device to be processed with information included in the request and a physical device corresponding to the relevant logical device, releases a definition of a host path of the logical device, and stops the physical device.