The etherCAT will likely recover faster than the drives. The drives have to recharge their DC bus, control power, and exit their alarm state prior to being useful, whereas the etherCAT modules will get power, clear alarms, and connect on receiving power again. I am making the assumption that the same backup power goes to both pieces of equipment. But for thorough safety/uptime, following Michael's advice and adding a UPS for the etherCAT modules would be best. The same would be true of ethernet as well.