Prio to Linux Mint 17.0 I used to run my laptop with the lid shut and used just the HDMI monitor connected to it when I was in the office. When I upgraded to Linux Mint 17.0 closing the lid caused both the laptop monitor and HDMI monitor to go off.
After finally getting annoyed with the laptop monitor changing in the peripheral of my vision I decided to fix it.
After a little googling I found http://forum.siduction.org/index.php?topic=4531.0
I restarted and logged in and now I can shut the laptop lid without the HDMI monitor going off.