OWC has the OWC Thunderbay 4 with free spare 4TB HDD in tray on special this weekend for $1149 (about $350 off).
It’s the RAID-5 edition with an extra (cold spare) drive. That’s five 4TB hard drives total. Use the cold spare as a backup in the meantime—no need to let it sit in drawer (hint: buy a 2nd empty Thunderbay).
RAID-5 is fault-tolerant meaning it can survive the failure of one of the drives: swap it for the spare and keep working.
You can run the unit as a single 12TB RAID-5 volume, or (my preferred approach) partition it into three separate 4TB RAID-5 volumes. Or not use RAID at all, and use the drives as single non-RAID drives.
See the MPG review of the OWC Thunderbay 4. MPG has five of these units—favorite storage device ever.
UPDATE: OWC also has the 5.0TB Toshiba drives with 3 year warranty for $163.99. Perfect for the Thunderbay, and OWC’s 90-day DOA replacement guarantee* beats anyone out there that I know of.
* If a new drive fails within 90 days OWC will replace it with a new one.