If you are looking for a neat solution to adding more external HDD storage, consider the Toaster 4 Bay USB3 External HDD enclosure. Forget about the Drobo. The 4 bay USB 3 Toaster TT400U3 at around RM430, is only 10% of the cost of a Drobo, but you will have to buy your own HDDs. It has 4 bays of docking receptacles which can support upto 4x3TB HDDs and it connects to your PC by a fast USB 3 port which is 10x faster than the usual USB2.
BTW, each standard 3 Tb HDD is around RM500 to RM600. I've been using the Toaster for a while now, and frequently changing HDDs with absolutely no problems. It takes about 5 seconds to change HDDs, although you should eject the HDD first, switch off the unit, and then install or remove HDDs. Its completely plug and Play on a Widows 7 OS. And the advantage for me is that I need only 1 USB3 point through which I can connect upto 12 Tb of HDD space, compared to one USB port for each of the single free standing Western Digital 2 and 3 Tb My Book essential HDD external enclosures that I've been using. As I have 2 USB3 ports on my PC, I can have upto 24 TB of HDD space (3Tb x 4x2)connected externally at any one time through two fast USB3 ports. Over time I have been upgrading the 6 internal HDDs I have to the newer 3 Tb ones. So with the internal HDD space already maxed out at 6 x 3 Tb = 18 Tb, I can now have at least 18+24 = 42 TB HDD space live at any one time, quite enough (for the moment) for my 600,000 RAW picture files, and at least one set of backups, performed automatically and periodically by Norton 360. Norton 360 automatically adds newer files into my back up sets to a set routine.
Here are some pictures of my Toaster Hard Disk solution. Bought from Computer War in SS2. Mention my name and you might get a discount, or at least I'll get a bigger discount when I buy my next 3Tb HDD.
If you need an External Hard Disk Solution, take a look at the Toaster TT400U3 HDD docking station.