Since about two years, after performing many many upgrades since the sidux/aptosid period, I cannot any more mount external drives (hard disks or USB sticks) by clicking on the xfdesktop icon or via thunar. I just get "Not authorized to perform operation", but that's insufficient for me to find out what's going on. Things work (not so surprisingly) by editing /etc/fstab and mounting via the command line in a terminal, so this is not a severe problem, just a bit annoying. I suspect something is wrong or missing in the policykit or udisks2 settings, but in spite of an extensive search through these files, I have not been able to find an appropriate solution ... Any clue ?