Ubuntu 12.04 Unity: Black screen with cursor only, when external display is connected before booting laptop

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

Side effect of this solution: I lose all of my Unity appearance settings.

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?

I've received this black screen with cursor myself (I use 4 monitors, and so unplugging them happens often).

However my fix was to correctly install my GPU's drivers.

Since your laptop apparently has an Intel HD 3000 series GPU have you tried installing up-to-date drivers? The Additional Drivers tab in your Software & Updates part of System Settings would be a good place to start.