The display drivers will sometimes not play well with the HDMI output to your external monitor in a linux hybrid graphics laptop. Some models have HDMI output connected to the discrete nvidia or ATI card, whereas some have it connected to the Intel integrated card.
If you are having problems with HDMI output, please submit a bug report to the xorg package. Open a terminal window and type:
(Choose 'Yes' if you see a warning about end of technical support)
Then type a description of your problem in the launchpad web page that will appear in your browser.
If you cannot follow these steps while the laptop is connected to the external screen, please still submit the bug report with the laptop disconnected from the external screen, since most useful information will still be processed.
Once submitted, please send an email to the launchpad hybrid-graphics-linux mailing list with the launchpad bug id. It will be something like: