Triple post (and please try not to do it in the future, double posting is frowned upon enough
)!
EDIT: And threadsurrection!
Anyway, the 5gb/6gb units use 1inch microdrives (branded "ST1"). I believe that physically swapping H10 drives is possible (people have opened them up, taken the drives out, but then just replaced the original one), but there might be issues with firmware, etc, and I don't think anyone has ever tried it.
The 20gb H10s use 1.8inch drives, which are the standard for DAPs. These are also used in the H1xx series, H3xx series, PMP, iPod, Gigabeat, etc (the list goes on)...