Just today on a consulting call for a client who generates 5-6 terabytes of images per year, I recommended the new OWC QX2 as a RAID-5 external storage device with an astonishing 9TB of usable capacity, connected to a robust MPG Pro Workstation taking the primary load, and the QX2 serving for overflow storage capacity (migration of older images and projects).
Backup is critical, so at least one more unit should be used to backup the main one. Never, ever assume that RAID protects you entirely, because theft, fire or flood or lightning can neutralize the entire system; keep at least one backup off-site (stored well away from the computer), and I recommend three, to be cycled/rotated, oldest one updated once a week or similar interval.
RAID-5 tolerates one drive failure, so with the 4-drive QX2, you get the capacity of 3 drives using 4 total drives (four 3TB drives = 9TB usable capacity with RAID-5). Keep a spare drive on hand so that any failure can be dealt with immediately, by inserting a replacement hard drive.
See my review of the QX2, and here is the page to get the QX2. The 12TB model (9TB as RAID-5) is only about $1400, a ridiculously aggressive price for storage capacity that only a few years ago was strictly for large enterprises.
Why get one? Unlike most other brand solutions that are shipped with little or no testing, all QX2 units endure a multi-hour burn-in certification to ensure the unit is operating properly and that the installed drives are ready for demanding use. For the 9TB unit, the procedure is about a 20 hour burn-in process. Definitely an uncommon committment to quality control.
For those looking for even greater fault tolerance or capacity (at greater cost), see my review of the CiDesign iStoragePro.