1U Backup Appliance -- purpose-built backup target, not a primary-storage device. Everything about its spec sheet is tuned for "the thing a real share mirrors to," not for serving a demanding client directly.
8 disks, 100TB usable, RAID6, 20000 IOPS, 8GB cache, 50% built-in redundancy chance -- the highest built-in redundancy of any NAS-category device, at the cost of the lowest IOPS/cache numbers among the three NAS tiers.
Cheaper per-TB than the RAID tiers by deliberately trading away IOPS/cache for it -- exactly the right trade for a device whose whole job is sitting there holding a synced copy, not answering live client traffic.
The natural home for a mirror share. Open this appliance's inspector, pick the NAS holding the data, then pick a client shared on it — or Everything on this NAS to mirror every client it serves at once. The share is created, sized and linked for you, and the disaster-recovery tick then keeps usedBlocks and virtual files in sync whenever both ends are powered, linked and addressed.
Note that a NAS can only back up the client shares another NAS hosts, never that NAS's own storage — a NAS has no share of itself. Mirrors are also sized against what this appliance actually has free, so pointing a large NAS at a small one fills it and stops rather than claiming capacity it doesn't have.
Tip: a mirror only protects you if it's actually current -- the status next to a mirrored share (Protected / Stale / Can't sync, plus the reason) is the thing to check before assuming a fault won't cost you data, not just the presence of a configured target. A confirmed mirror counts as cover for 60 seconds, so a brief blip isn't lost protection.
See also: the Game Systems manual's Data Management section for backup mirrors and disaster recovery.
Takes the Redundant PSU upgrade ($1800, Inspector → Upgrades): it adds a second power input, PSU2. Wire PSU2 to a DIFFERENT PDU or UPS than PSU1 and this device keeps running when either one dies. Two feeds from the SAME PDU protects nothing -- the redundancy panel simulates that PDU failing and watches the device fail with it regardless of cable count. The spare idles at zero draw, so it costs nothing extra on the power bill. See Redundancy & Failover.