Disable the Internal Screen on a MacBook Pro or Air in OS X Mavericks
Some MacBook Pro or Air users may want to disable their internal screen when the laptop is connected to an external display, this is generally achieved in two ways but ever since Mac OS X 10.7, 10.8, and 10.9, the internal screen is more persistent and wants to stay on.
To disable the internal screen for OS X Lion, Mountain Lion, and OS X Mavericks based Mac laptops, launch the Terminal and enter the following command:
sudo nvram boot-args="iog=0x0"
You’ll need to reboot for changes to take effect, and the internal display will then be completely disabled regardless of whether the Mac is open or closed.
To undo this, you can go back to the Terminal and enter:
sudo nvram -d boot-args
Then reboot again, or you can just zap PRAM by holding down Command+Option+P+R during reboot, which clears out the boot-args as well. If you were to disconnect the MacBook Pro from an external video source, zapping the PRAM would be how you’d want to reenable the internal display.
This is the opposite of “clamshell mode” – where the Mac laptop is closed and the screen is still turned on. Clamshell can look nice, but without adequate air flow the Mac may overheat, thus running the computer with the display open is recommended. If you’re going to do this, be sure to set the primary display so that the menubar, Dock, and alert windows go to the proper screen.
Thanks to Marcus for the tip via a thread on Apple Discussions