Maybe the xbox 360 OS is smart enough to use the hard drive when it is here.
if you don't have it, the OS knows it, and won't use any HD cache.
If you have it, the OS uses the HD cache in a sort of Plug and Play
It's certainly a good idea for selling more hard drive if it can reduce loading times, help in BC, replaces a memory card, and is able to store all your media.