I’ve recommended the OWC Thunderbay 4 for a long time, and I use five of them.
Now OWC is offering a deal: the 20TB Thunderbay 4 RAID-5 edition with a spare hard drive.
The idea with RAID-5 is fault tolerance. RAID-5 can be thought of as “striping with parity”; it offers the high performance of RAID-0 striping with fault tolerance. A 20TB RAID-5 offers 15TB of usable storage (the equivalent of one drive is used for parity data).
That spare hard drive is a cold spare: should a drive fail, the unit just keeps working (degrading to a RAID-0 stripe); the cold spare is swapped out for the failed drive and the RAID-5 rebuilds while you can keep right on working. Very cool.
Prefer not to use RAID-5? The Thunderbay 4 can quickly be reconfigured for single drives (4 of them), RAID-0, RAID-1, or RAID-10.