Okay, I've purchased a 100GB used drive through eBay.
Mmm.. dead end again. The drive turned out to have a ZIF interface rather than the advertised IDE connector. Returning to seller for full refund (nice seller!).
And now hunting again..
I originally wanted to upgrade for two reasons: the factory drive is much slower than current model drives, and it's too small to hold the ever-increasing RAW files generated by my camera over a 3-4 week holiday.
If it turns out that I cannot get a larger (~100GB) drive for it, then it may be time to go instead for speed alone, and put a fast UDMA CF card in there.
A decent 32GB card seems to run close to $200 right now, and a very fast one for about double that.
But I'd still rather find a larger hard drive that fits. I can fit a 5mm drive + ZIF adapter into it no problem, but there's just no space for an 8mm drive + ZIF adapter. And it takes 8mm non-ZIF drives just fine as well.
???