Can't mount external drives or shutdown in XFCE on Debian Wheezy Live

As I mentioned in my review of XFCE Live on Debian Wheezy, I wasn't able to access external drives or shut the computer down properly. Plugging in and trying to access a USB drive produced this message.

Failed to mount [drive] not authorized

There 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 lightdm

It'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.