Yesterday I got a Drobo to use with Time Machine. I only got the USB 2.0 model, since I have it connected to my Airport Extreme 802.11n base station. I ordered it with 2x 1TB drives.
The Drobo is perfect for Time Machine backups, since they grow continuously. When you run out of space, you can simply add another drive to the Drobo to increase its storage capacity. Adding a drive to the Drobo is very easy – simply snap off the faceplate and insert a 3.5″ SATA drive into one of the empty drive bays until it ‘snaps’ into place. You don’t even have to shut down the Drobo. It will immediately recognize the new drive and start using it.
The Drobo has some advantages over traditional RAID devices, like my TeraStation. Most notably, it doesn’t need any setup when you add a new drive – it simply reconfigures itself and starts using the additional storage. The TeraStation also doesn’t support Time Machine without some ugly hacks that don’t work reliably. With all 4 drive bays filled with 250G drives, the TeraStation is a lot more difficult to expand.