http://ubuntuforums.org/showthread.php?t=83973
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se placer en [[console_native|console native]], car le serveur X va être fermé :
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sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf_backup
sudo /etc/init.d/gdm stop
sudo dpkg-reconfigure xserver-xorg
sudo /etc/init.d/gdm start