Provided is a storage system having a host system, a first storage controller and a second storage controller. The first storage controller has a primary logical volume formed from an actual volume, and a secondary logical volume formed from a virtual volume capable of forming a copy pair with the primary logical volume. The second storage controller has an external volume formed from an actual volume to be mapped to the secondary logical volume. The host system has device data showing the mapping relationship between the external volume and the secondary logical volume.