Abstract: Data migration is made to be performed in short time without actual data move and independent from the capacity of a magnetic disk volume. As viewed from an upper hierarchical system, “new physical drive installation or addition” can be performed easily, rapidly and safely. If a physical address is to be changed after a host generates or updates data in a magnetic disk volume, a service processor is made to change only a correspondence between physical and logical addresses. The physical address can therefore be changed by retaining the data written in a magnetic disk. Accordingly, if there is an idle area in a physical drive, as viewed from an upper hierarchical system, “physical drive installation or addition” can be performed easily.