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Whenever I connect an external display to the DSub port of my laptop before booting into Ubuntu, something goes wrong in Unity to cause the exact same problem as described here: After logging in, there's a black screen and my cursor, nothing else! in Ubuntu 12.10 Rebooting without the display connected does not fix it.

A once-off fix is to do this:

sudo apt-get install ppa-purge
sudo ppa-purge ppa:unity-team/staging

sudo apt-get update
sudo apt-get install --reinstall ubuntu-desktop
sudo apt-get install unity

or, much quicker, this:

unity --reset
sudo service lightdm restart

My problem, however, is that I need to boot with the external display connected quite often. Who knows what goes wrong inside Unity and/or how to fix it permanently?

share|improve this question
It might help to know what hardware you are using. Some laptops have a BIOS setting to enable/disable external. –  Elder Geek Jan 7 at 2:12

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