The backup copy (or copies) has the same issues/concerns as the original array has. Even with RAID(6) one needs a backup. The backup can be a RAID, mhddfs or IBM reel-to-reel. It's a separate filesystem, and can be designed however one likes.

For me, I'm using non-redundant mhddfs again to bond four 2TB drives. These are just media files -- a pain to replace, but not a catastrophe if lost. So I'm happy with a single backup copy of everything, effectively giving me RAID1 on a per-file basis (one copy on the live system, one on the backup), but without the headaches of RAID.

Mechanical drives seldom die outright without warning. The failure mode is more typically bad sectors accumulating (dust/dirt scratching the platters), so it might lose a few files, but probably not the entire filesystem. And with mhddfs the damage is limited to only that one drive, not all drives.

RAID *needs* RAID, because it throws everything into a single large basket, where a single drive failure loses everything unless redundant drives are configured. And RAID multiplies the probability of total failure by the number of drives.. making a catastrophe more likely.

Cheers