I have 2 internals and one external on my DAW. I do seem to recall a spare HD connector in the box should I decide to drop another internal into the box. Right now, I have more than sufficient storage real estate for years to come..... even given a 1GB per song file size, which is generally not the case.
C has XP and the programs I run (audio DAW stuff only)
E has the huge sample libraries and storage folders for my projects (archived)
F/external is a backup for C image and E projects and libraries.
I allow X1 to use C for current and recent project storage. I move them to E when I'm pretty sure I'm done working on them.... I don't zip them, I simply drag the folders.