Which RAID level should I use in a two-bay NAS?

A two-bay chassis offers a mirror, a stripe with no redundancy, a joined volume, or two independent volumes. The mirror is the only one of the four that survives a drive failure, and it presents the capacity of one drive.

Two bays is the commonest chassis in a home, and it offers exactly four arrangements. Each answers a different question.

The four arrangements

Mirror. Both drives hold the same content. One can fail without the volume becoming unreadable. Usable capacity is one drive.

Stripe. Data is spread across both drives for speed, with no redundancy of any kind. Usable capacity is both drives, and the failure of either one loses the whole volume — the risk is therefore higher than with a single drive, since there are now two things that can end it.

Joined volume. The two drives are presented as one larger space, filled in sequence rather than striped. Usable capacity is both drives, and a failure loses at least what was on the failed member.

Two independent volumes. No array at all. Each drive is its own space, and a failure loses that drive’s contents only.

Choosing between them

If the machine has to stay available, the mirror is the answer, because it is the only level in a two-bay chassis that survives a failure. This covers most households: the machine keeps serving, and a replacement drive is an errand.

If capacity is the constraint and everything on the machine exists elsewhere too, a joined volume gives the full space. This is a reasonable choice for media that has been copied from somewhere, and an unreasonable one for anything that has not.

If the two drives are doing different jobs, keep them independent. One drive as the working share and the other as a scheduled copy of it is a genuinely useful arrangement, because the copy has history — yesterday’s version of a file still exists on it — which no mirror provides.

Striping is rarely the right answer in a two-bay network enclosure. The network link is generally the narrower part of the path, so the speed the level buys is spent before it arrives, and the failure exposure has doubled for it.

What none of the four provides

None of them is a backup, and the mirror least deserves to be mistaken for one because it looks the most like a copy. Both members receive the same deletion, the same hostile encryption and the same enclosure fault, and both are in the same room during the same theft or fire.

Whatever level is chosen, the question of where the second copy lives is still open, and it is answered on different hardware.

Two lines to read before the drives arrive

The maker’s migration table, which says which level changes are possible in place. And the maker’s compatibility list with its maximum capacity per bay, because the largest drives on sale are not always supported by a given model. Both are published, and both are considerably cheaper to read than to discover.

The manufacturer’s own documentation comes first, and this page is not a substitute for it. Every rate, capacity, class figure, endurance rating, port speed and power figure on this page is reported from the manufacturer's own documentation, with its unit written as published, and named as such — none of it is measured here. A stated capacity is reported as the maker counts it, and is never restated in another unit, because the two are not the same figure. And the second statement repeated rather than assumed: a RAID level is not a backup. It keeps an array readable through the failure of a drive, and it does nothing about a file deleted by mistake, an array encrypted by ransomware, a theft or a fire. A second copy somewhere else is a separate decision, and nothing on this site describes a machine as a place where files are safe.