I am trying to make my own Live USB and I have successfully generated an ISO file with a newly installed, configured and customized Xubuntu 12.04 LTS installation. My problem is that, no matter I boot the ISO in VirtualBox or in GRUB with loopback, it just cannot start the lightdm. When booting, I can see the log messages on the screen, it stucks at Stopping System V compablities or Configuring Network security. And tty7 is frozen... If I switch to tty1, I can get a logged-in shell as ubuntu@ubuntu. The weird thing is: When I type sudo start lightdm or just sudo lightdm, it will switch to tty7 and the screen flashes. Then nothing happened. Return to tty1 and I can see lightdm running, process xxxx. But the process does not exist. It was just crashed immediately I think. (That's why the screen flashes.) However, when I type startx, I can get into the desktop! That's amazing for me. I am not very clear about the relationship between X Server and Display Manager, but I think lightdm is running when I see the desktop! Then, what's wrong with sudo start lightdm? I use this command every time I power on my laptop since I have a text parameter added in grub.cfg. It never "crashed immediately". I need to use sudo start lightdm because it gets me into "Xubuntu Session" instead of "Xfce Session", the prior is more beautiful... Could anyone help?