OS - Operating System! Lmao!!!
I get you now, but that sounds like alot of hassle and complicated..well for me anyway!
I think i'll just buy that 400GB hard drive, an external enclosure and use it as an external hard drive. I take it you get the cables ect when you buy the external enclosure, it just plugs into your USB port right?
Then i will be able to transfer all my files over to the external hard drive so i have a back-up.