I found the LaCie Thunderbolt, which comes with high quality Seagate DX 5x4TB drives [20TB total] for
£1425 (perhaps less if shopping around further).
LaCie supply the Seagate (Seagate now own LaCie)
DX "desktop premium" (7200rpm) vs. the standard
DM "desktop mainstream" (5900rpm) ones, so they are the best quality before you pay double for Enterprise drives.
The Drobo
you personally can get at £450, then add 5x4TB drives [20TB total] (cheapest I found was around £175 each for only similar
DM quality = total £875), + 64GB mSATA is around £75 (as Drobo suggest), thus grand total =
£1400.
And one of those drives would be for the
proprietary BeyondRaid parity redundancy, then Drobo overhead, makes it only 14.52TB (according to their checker here:
http://www.drobo.com/products/capacity-calculator ).
The LaCie is therefore effectively the same price with more space, and you get R0 (700Mbyte/s) performance, rather than the Drobo's (250Mbyte/s) slow performance.
(read the professional reviews online or even Amazon ones please, not hear-say quoted above: mSATA drive does not make these much faster than without adding it, just adds data-transfer reliability.)
Sure the Drobo can do rebuilds on drive failures, but whichever of these you go for you also MUST have a
separate backup, as if the Drobo or LaCie set-up failed completely, you'd lose entirely
everything.
You could go for a different slower backup to the LaCie clone suggestion of course, but having just one multi-drive enclosure with no local backup solution, is asking for trouble.
Trust me, I learnt this lesson the hard way I had only a single multi-drive R5 USB2 set-up a couple of years ago which failed entirely: lost everything.
Don't go there.