A method of operating a storage system comprises detecting a cut in an external power supply, switching to a local power supply, preventing receipt of input/output commands, copying content of cache memory to a local storage device and marking the content of the cache memory that has been copied to the local storage device. When a resumption of the external power supply is detected, the method continues by charging the local power supply, copying the content of the local storage device to the cache memory, processing the content of the cache memory with respect to at least one storage volume and receiving input/output commands. When detecting a second cut in the external power supply, the system switches to the local power supply, prevents receipt of input/output commands, and copies to the local storage device only the content of the cache memory that is not marked as present.