Patent ID: 7421553

Claim:
A storage system in which a main storage system is connected to a sub-storage system, the sort of which is different from the sort of said main storage system, for transmitting/receiving data between an upper grade apparatus and said storage system, wherein said main storage device system comprises: a receiving unit for receiving a data writing request from said upper grade apparatus; a data holding unit for holding data which is required to be written in response to said received data writing request; a transferring unit for transferring the data which is required to be written to said sub-storage system in response to said received data writing request; a data guaranteeing unit for guaranteeing whether or not said data written to said sub-storage system corresponds to correct data; wherein the data guaranteeing unit stores the data transferred to the sub-storage system at least until the guarantee and, if said data written to said sub-storage system corresponds to incorrect data, the transferring unit again transfers the data to said sub-storage system; and a data guarantee selecting unit for controlling the operation of said data guaranteeing unit; wherein said data guaranteeing unit is provided with a plurality of data guarantee modes, including at least one mode in which a guarantee code is used and at least one mode in which a guarantee code is not used; wherein in a case where a data guarantee mode selected by a user is a mode in which a guarantee code is not used, a data guaranteeing operation is executed in such a manner that the data which is required to be written is transferred to said sub-storage device so as to be held therein, said data held in said sub-storage device is read out, and said read data is compared with the data held in said main storage system.