You should use a combination of surge and battery backup for external backup drives, especially if the drives are the spinning kind. Although the arm will automatically park during a power failure you still could lose data:
"Although hardware damage should not occur, data loss is still possible. If writing is in progress, the hard drive will finish writing the current sector before retracting to the parking area so that sectors are never corrupted as a direct result of power failure. However, because data that has recently been sent to the drive may not have been committed to the platters (they are briefly held in the onboard cache for performance), that data may be lost on a power failure."
A small APC or other name brand battery backup is all you need, you don't need to overthink or over engineer this solution.