the reason RAID is not backup, aside from snapshotting, is colocated drives (same faulty machine, same flammable location, same malicious thunderhead) have a far higher chance of failing at the same time due to evnironmental effects --- hence the decision to have half the drives stored remotely. having a third copy, stored cold (blu-rays are a good idea) helps, but only if its failure is sufficiently independent af the others to make a real dent in the probabilities (e.g., a safe-deposit box or a friend's house, not where any of the drives are!)