Macos – Turn off the MacBook Pro’s display with the lid open while using an external monitor on OS X 10.10 Yosemite

http://www.cultofmac.com/176329/turn-off-internal-lcd-display-of-your-macbook-air-os-x-tips/

You can try these, Tip 1 has worked for me however you must close the lid during start-up/reboot before the logo appears. Once the login screen appears you can open the lid and the screen should be off. Can check with the brightness control buttons.

This is not an ideal solution as closing/opening the lid every time can be quite bothersome but it works (more info below). An option from Apple to turn this off with external monitors connected would be ideal.

Link: http://gizmodo.com/5938452/a-trick-to-make-using-an-external-monitor-with-your-macbook-way-better

Here is a terminal command thats fairly simple that will allow you to keep the lid open for adequate cooling but while using the external display only. I've been using this method with my Thunderbolt display and MacBook Air since the beta of Mountain Lion came out and it works perfect. Has all the same characteristics of good old clamshell mode. Be sure to keep this written down someplace in the off chance you need to re-install, do a PRAM reset, or just want to undo it.

To execute in Terminal:

sudo nvram boot-args="iog=0x0"

To undo in Terminal:

sudo nvram -d boot-args

Once you type it into terminal I believe you need to enter your password. I then restart my machine. Now the TRICK is to either restart your machine with the lid already closed (hit restart then slam the lid!) OR turn the machine on for the first time (then quickly slam the lid!) once you are past the login screen you can open the lid.

ALSO: if the machine is asleep, and you simply forget and open the lid, it will wake up both displays. So just remember to wake the machine up with an external bluetooth mouse or keyboard.