I had the OS X disk automount option enabled so that my external FireWire disks were mounted when no user was logged into the GUI. However, it created a problem of somesort in that none of my disk images would mount any more. I had set it by running the following command in the Terminal:
sudo defaults write /Library/Preferences/SystemConfiguration/\
autodiskmount AutomountDisksWithoutUserLogin -bool true
You should be able to prevent automount in the future by just changing true to false at the end of the above command. But you may also want to remove (after verifying its contents) the following file: /Library: Preferences: SystemConfiguration: autodiskmount.plist -- you'll need sudo to do so.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050326210151430