I don't really know much about what I'm talking about with those 12gb systems...

But I would imagine that regardless of how much space you have on an External drive. The system is still going to display to you the amount you have on your internal drive when you go to system settings. Just save your files to your external hard drive. Or save them to the internal, then copy them to the external before deleting them as needed.

Sorry if I'm completely wrong.