Arrangements for transfer and replication of data. A host computer retains a transfer volume management information including HDD identifiers corresponding to a plurality of primary volumes and identifiers of at least one of transfer volumes, wherein, from a plurality of volumes in a first storage device, the host computer extracts a reserve volume as an additional transfer volume, if a capacity usage of the at least one of the transfer volumes reaches or goes above a predetermined level, wherein the host computer transmits an allocation request about the additional transfer volume, wherein, after receiving the allocation request, the first storage device implements the reserve volume as the additional transfer volume corresponding to the at least one of transfer volumes for transfer of write data to the second storage device.