HDDs for the current Xbox 360 models aren't compatible with the redesigned line- they physically won't fit. It's so the 4GB can upgrade anytime.
This isn't a surprising move as Microsoft has eventually made all their HDD offerings for Xbox 360 available as standalone purchases. The new HDDs slot into the base of the redesigned Xbox 360 console, unlike the now 'old retro' units.
Have you picked up an Xbox 360 S?