One nice thing about the "2 different forms of media" rule is that it would protect you against bad batches of hard drives. Sometimes, when people buy multiple

One nice thing about the "2 different forms of media" rule is that it would protect you against bad batches of hard drives. Sometimes, when people buy multiple drives at once for servers, they wind up with drives from the same batch. If the drives are subtly defective, and if they're always run under identical conditions (such as in a RAID array), then I've seen multiple drives fail within days of each other.