Storage system having dynamic volume allocation function

A storage system coupled to a computer and an external storage system, the storage system comprising:

• a first storage controller managing a target device to be accessed by the computer, a first logical device allocated to the first target device;
• and
• an external storage connection adapter connected to the external storage system, which includes a second storage controller and a plurality of physical storage devices storing data sent from the storage system, the second storage controller managing a first external target device and a second external target device both of which are configured by the physical storage devices,
• wherein the first storage controller allocates the first external target device in the external storage system to the first logical device, and
• wherein the first storage controller detaches allocation of the first external target device from the first logical storage device and then attaches the second external target device to the first logical storage device.

A computer system comprising:

• a storage system coupled to a computer and including a first storage controller managing a target device to be accessed by the computer, a first logical device being allocated to the first target device;
• and
• an external storage system coupled to the storage system, and including a second storage controller and a plurality of physical storage devices storing data sent from the storage system, the second storage controller managing a first external target device and a second external target device both of which are configured by the physical storage devices,
• wherein the first storage controller allocates the first external target device in the external storage system to the first logical device, and
• wherein the first storage controller detaches allocation of the first external target device from the first logical storage device and then attaches the second external target device to the first logical storage device so that the computer can access the second external target device in the external storage system through the first target device instead of the first external target device.