I took a look inside the shield awhile ago. It seems like the hard drive is on a special connector that is on a separate board that has the rear ports on it. It seems like the 500 gb models have the connectors soldered on the board whereas the 16gb don't have the connector at all, only pads on that board.
TL;DR: If you want to upgrade you're SOL.
!&&* even the hard drive format is special and isn't recognized by windows. Haven't tried Mac yet.