Drobo are very expensive but they are very cool. Basically a drobo is an external HDD attached to a raid controller and server in one box. The main benefit of using a drobo over a dumb external drive is the drobo is manageable and scale able If you want to upgrade a HDD in the raid u just pull it out and replace it with a larger one. Drobo handles moving the data and moving it back all without bothering you. A dumb HDD you would need to move the data off the drive, replace it and move all the data back. This can be hard to do with large drives like 2tb. drobo arent fast or bigger they are just smarter and easier to manage.
I dont own a drobo. they are too expensive for me.