Looked at the PCManFM settings & config file but nothing untoward there. Interestingly, when I look at the HDD with the gnome-disk-utility it is unmounted, although PCManFM is set to auto mount. There's no problem looking at other usbs or cds on the system which auto mount fine. I tried to mount the HDD with gnome-disk-utility but it tells me "Not authorized to perform operation (udisks-error-quark, 4)". Seems like there's the need for some permissions but not sure how to get them or why they're not there in the first place. Any suggestions?
Those links you gave are very interesting. I'll give them a good looking at.