usb hard drive problem

I took an 80gb hard drive out of a dead system (hd was ok, mobo went bad) and installed it in an external box with a usb connection. I want to use it as a backup hard drive. problem is, my winxp sp2 box will only see 31gb of the drive. anybody got a clue for recovering the whole drive short of tearing the pc apart and installing it in the box?