The Toshiba MK8007GAH (80GB) is the largest that has the right connector, and they are quite hard to find. The Toshiba MK1214GAH (120GB) works fine, but you need the latest small ZIF adapter
* to fit it in the case without excessive strain. I also have a Samsung 120GB - this works, but at a much slower speed than the Toshiba - something is not right, I think.
I haven't tried any larger ones yet, but a special build of Rockbox is required to use disks of over 137GB or with larger sector sizes (don't know the exact details); and if the bootloader needs changing, that carries a danger of bricking the device.
* The one described earlier in that thread is still too big for the H3x0