I have a drobo too, and this thing is pretty sweet. It uses magical robotic powers to give you complete redundancy without having 2x the storage capacity. for example, if you have 4TB of storage loaded into it, you get 3 TB of usable protected space. Not sure how it works. It does have a fan though, which makes a bit of noise.
Also, I would recomend getting the most expensive drive by whichever manufactuerer you choose. The "enterprise" drives are the ones to go with, because they are made to last. Also, within each manufacturer, the more expensive a drive is, the longer the warranty is, which I have heard is also a good sign, because they don't want to have to replace many drives.
I have heard good and bad things about every brand though, so I have no idea which is best. I have a bunch of WD drives, a Hitachi, and a Seagate I believe. ??