hzn: Indeed. While people celebrate the RiscPC case design, it's actually dreadfully thought-out inside (assuming it was thought about at all.) Other than the issue of the case being plastic and thus not acting as a heat sink (of course, originally this wasn't a problem, but modern hard discs kick out lots more heat, as well as new podules and CPU cards doing this), the things that annoyed me most was the faff you had to go to when you had more than one slice and you wanted to replace something, and the entire waste of the space above the CPUs in the extra slices.
Putting the hard drive in a podule bay, however, is an excellent idea. Although I would go to some effort to make sure it is secured in place by more than just gravity.