I'm in a vaguely similar situation and have done a bit of research on 2tb internal drives, for what it's worth.
In total I've had seven WD drives over 8yrs (two OEM fitment in Dell desktops, two retro fitted drives into said desktops, and three external "Mybook" efforts) and, finger's crossed, have never suffered a failure.
Based on this experience I started looking at Western Digital drives and later also had a look at Seagate. Since historically Maxtor and Hitachi don't have a good rep. for drive longevity, I didn't bother investigating them. Havn't looked into Samsung either, although will do on the recommendations in this thread.
The first thing to note is that drive characteristics vary with capacity within a particular model range; so just because a 2tb is recommended, doesn't mean a 1.5tb in the same range will be as good. Best to decide what capacity you need, then research drives of that capacity, rather than researching a range as a whole.
I looked at WD Caviar Green drives initially due to their cost / low power consumption and low noise. Most tests confirmed these characteristics, but also showed that they were quite a bit slower than WD Cavier Black drives (IIRC the black drives were about 40% faster, although I can't find the site now). However, Black drives are costlier, use more power and create more noise.
Currently a 2tb WD Green drive with 64mb cache can be had from Amazon for about £140 while the Black equivalent is about £240. The Black also has a 5yr warranty as opposed to 3yr for the Green.
It appears that in terms of price per Gb the 2tb drives are about in the middle of their respective product ranges, being cheaper than the sub-1tb models, and more expensive than the 1 and 1.5tb models.
I abused Google for reliability information, and it appears that the Green drives have a reasonably high failure rate, however I think a lot of these were DOA due to poor packaging used for mail order products (this appeared to apply to all drives regardless of make, to a certain degree).
I looked at the Seagate Barracuda LP (the equivalent of the 2tb WD Green drive), and whilst the Seagate's performance was reputidly a bit better, it has an abysmal rep for failure so I'll be steering clear of them. While historically Seagate have always had a good name for reliability, it appears that they've gone rapidly downhill since they procured Maxtor.
In summary I was left somewhat unimpressed by all of the drives in terms of reliability, however it appears that the Cavier Black is the best of the lot, if you can stomach the price and power consumption (which I personally can't - a grand for four drives!). Next are the Caviar Greens, and then the Seagates.
Finally, some things worth bearing in mind are; while reliability is obviously paramount, for backup and data storage, how important is drive speed?
Also, referring to RobertP's retrodata link, it's probably worth considering the power consumption and thermal properties of drives with respect to how they'll be stored. Does the Drobo have integrated fans? If not, that's a lot of energy being dissipated in a relatively small, enclosed space, and low power consumption drives might be the best bet for this application.
Good luck finding a solution - Lack of funds means I won't be buying owt in the near future, which is probably a good thing as hopefully over time reliability should improve and price should fall..
or Western Digital Caviar Green.....
