1. A storage system, comprising: a controller coupled to a host computer or an information processing device and controlling to transfer data sent from said host computer to a storage region; a first logical volume related to said storage region; a plurality of second logical volumes being used to maintain consistency among data stored in said second logical volumes and being used to store a copy of data stored in said first logical volume; a third logical volume being used to store a copy of data stored in said first logical volume; and a plurality of disk drive units having a plurality of disk drives which have said storage region; wherein said controller processes the steps of: receiving a split command sent from said host computer or said information processing device, receiving a read/write command sent from said host computer to said first logical volume or one of said second logical volumes after the step of receiving said split command, copying data related said read/write command from said first logical volume to said third logical volume, processing said read/write command by using said first logical volume, said one of said second logical volumes or said third logical volume, and replying a reply related to said read/write command to said host computer while maintaining consistency among data stored in said second logical volumes.