How about a DVD-R drive? 4.7GB at about $5 a disc. Not too bad, pricewise, and eminently swappable. Or you could go with a FireWire/USB/other swappable hard drive solution where you could swap the physical drive out of the connecting box and have hard drives lying around as removable media. Or, since both FireWire and USB support many devices, you could just keep adding new drives to your system when you run out of space, only using the one controller that you probably already have.