To those suggesting people buy Xbox 360 hard drives on ebay I must warn you that the secondary market is flooded with crappy Chinese knock-offs. If you read carefully sometimes they will say it's generic, but most of the time they will flat out claim it as a Microsoft drive. They look pretty much the same and they work, but often can't play Xbox original disc games. It has something to do with the way they are formatted. Official drives have a special partition for backwards compatibility and most of the knock-offs are missing it.
I'm cheap and I wanted a larger hard drive and I thought a used drive would be the way to go. Well, I had to send back 3 hard drives before breaking down and buying brand new one on Amazon.com before I could play my Xbox originals.
I consider myself lucky to have been able to argue my way to a refund three times. Don't trust the honesty of ebay sellers who sell knock off merchandise.
Anyway, I just wanted to warn you guys. If you have a 4gb model I recommend buying a new drive from a real retailer instead of trying your luck with a shady deal on ebay.
