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How do I get my external hard drive to work for my xbox360 slim?

I have a SimpleTech 1TB external hard drive and a 4GB slim 360 (just till the elite kinect is in on 11/04) and I would like to use my hard drive so I can play firefight on Halo Reach online. I don't want to spend 150$ on a 360 slim hard drive. Anyone know how to do this?