1. A storage system, comprising: a controller device coupled to a host computer 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 each 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 device processes the steps of: receiving a split command sent from said host computer, in accordance with said split command, said controller device starts to change to a first status in which a relationship between said first logical volume and said second logical volumes is split, receiving a read and/or write command sent from said host computer to said first logical volume after the step of receiving said split command, copying data related to said read and/or write command from said first logical volume to said third logical volume and then copying said data related to the read and/or write command saved in said third logical volume to said plurality of second logical volumes, processing said read and/or write command by using said first logical volume, said second logical volumes or said third logical volume, and replying a reply related to said read and/or write command to said host computer.