I have this very annoying probklem that I have not been able to solve. when I install gnome-core usb disks does not mount anymore. I get "not authorized".
Generally if I use gdm it works but if I use xbmc or startx it does not work.
I think the problem could be that the policykit is upgraded to polictkit-1 and any settings that are done for xbmc does not work anymore.
I have now found this: EDIT: This does not fix the usb mount problem. It fixes some other permission problems though. I have given up on xbmc and gnome for the moment. I use lxde with xfce and kde components instead.
(sudo apt-get install policykit-1 devicekit-power)
sudo nano /var/lib/polkit-1/localauthority/50-local.d/custom-actions.pkla
[Actions for xbmc user]