In a computer system having a computer, a storage system having storage units being coupled to the computer and for storing user data used by the computer, and a backup device being coupled to the computer and the storage system, a backup method is presented for obtaining a backup of the data stored in the storage unit to the backup device. The storage system holds the user data dually in a first and a second storage units. In backing up the user data, a split instruction for releasing the duplex state of the first and the second storage units is sent from the computer to the storage unit. In response to the split instruction, the storage system interrupts the reflection of the update of the user data to the first storage unit onto the second storage unit. Then, a copy instruction for copying the user data held in the second storage unit to the backup device is sent from the computer to the storage system. The storage system operates to transfer the user data inside of the second storage unit to the backup device in response to the copy instruction.