ok... Now I have a MK3006GAL inside
! it works perfectly... I think 30gb is the biggest size available: 40GB ones are thicker than 5mm (at least according to toshiba hdds, I don't know if other manufacturers produce 5mm 40gb hdd)
EDIT: I just realized that MK4007GAL is of the same series, but 40GB single plate (5mm)... but it doesn't mind, I think it's much harder to find and much more expensive, and 30gb is enough for me
Briefly, the compatible hdds are:
MK2006GAL - 20 GB (it's the unit the gmini comes with)
MK2004GAL - 20 GB (see below)
MK3006GAL - 30 GB
MK4007GAL - 40 GB
Other new models with perpendicular recording (see below)
seems like some units have the MK2004GAL inside, I've added it to the list above.
I also add that toshiba now (2008) produces other models with perpendicular recording that offer much larger capacities mantaining the 5mm thickness, for example i saw a 80gb model.
Remember to chek that the model you are considering has the right connector, not the zif/lif