For those of you that don't know what a mobile rack is, it is a drive bay that fits where a CD rom would fit in your desktop. Mount your hard drive(s) in the removable tray(s) and you can swap hard drives out of your computer using the 5.25 bay on your computer. The bay is connected to your motherboard, and the removeable tray is connected to your hard drive.
http://www.kingwin.com/mobileracks.asp
I have one hard drive that only has poker clients and clean software (e.g., PokerTracker), and nothing else is allowed to connect to the internet. When done playing, I swap the tray out with another tray that contains the drive that I use for everything else. Now, if I want, I can take or put my poker hard drive anywhere I want.