Failed to mount [drive] not authorizedThere are many posts about this problem on the internet. Reading around, it seems the problem is caused by the display manager not passing the required permissions to security services which control access and shutdown. I saw LightDM suggested in several places, but it was already installed.
In the end I found that dropping out of the desktop environment to the command line and running LightDM as root fixed the problem: XFCE started with access enabled.
To do this, press Ctrl+Alt+F1, then Ctrl+Alt+F2 to get a command prompt, then enter:
Sudo lightdmIt's quite possible that if XFCE is installed, LightDM will be set up to run and this problem won't exist, but I've only run XFCE from a Live USB, so I can't confirm this.