XBox Slim 4gig HDD options

I just picked up a 4gig slim and i cant play firefight or co op online in halo reach, Apparently i dont have a hard drive to do so.

Is it possible to use an old HDD, ive seen a few articles but that seems fishy? And would using an old HDD count as tampering?

Update: I just called microsoft and the person told me it would count as tampering, but i called again since the operator seemed like an idiot and another person who seemed more competent said he saw no problem with it.

Second Update: Unshelling your old Hard Drive will void its warranty (obviosly), it does work and it my xbox is not banned so this must be somewhat legit, i called xbox customer support again today and they said it was not violating the user agreement to do so.

Was there not enough storage space for it, or did you need an actual drive?

USB stick can be made to expand. I have a 4GB slim 360 also, and used one of my 8GB USBs lying around to add more space to the 360.