I wouldn't recommend the Drobo. If the hardware (the Drobo) fails, all your data is not accessible until you ship in the Drobo and get a replacement after a while. And it's a small company, so if you're out of warranty, your data is gone for good, there's no software to read the files.
Instead, buy two same sized USB hard drives, one for data, one for the backup (Time Machine, Super Duper, Carbon Copy Cloner) of the data disk. You can use OSX's built-in software RAID so one drive mirrors the other. Or you set up Time machine, that has the advantage that you can recover accidentally deleted stuff. If you go the Time Machine route, you can also use differently sized drives.
A decent 1500 GB Drive is not really expensive anymore, you can get a Seagate Freeagent Desk (don't bother with the more expensive Mac version) for about 130 Dollars. Those drives are reliable, very quiet an power efficient (they work with OSX's energy saving setting "put hard drive to sleep after x minutes"). Those drives also come with a native OSX diagnostic utility which tells you when a drive is starting to fail (S.M.A.R.T. technology plus some proprietary stuff that looks for corrupted files and sectors).