Two copies could be enough if the filing system has mechanisms for verifying file integrity, so never more than four copies in total - no point accumulating the

Two copies could be enough if the filing system has mechanisms for verifying file integrity, so never more than four copies in total - no point accumulating them. It's possible flash-based media could be left unused for two to three years before needing to be powered up, but I think this time-span gets shorter as the storage density increases.