My USB hard drives stopped automatically mounting on my XBMC Live installation. Fortunately, I found a fix as specified here:
http://forum.xbmc.org/showthread.php?t=71492
Apparently this was due to me installing Firefox which requires some Gnome components. Those Gnome apparently components caused the problem.
I ran:
sudo apt-get autoremove --purge firefox
rebooted, and the problem was fixed.
Note: