I spend few days fighting with broken screen in Acer Aspire 6930g. My friend forgot a password and I could't get it boot on external screen to do recovery mode. Acer did not reply for my emails. Fn + F5 only works in Windows, not when booting up.
Here is the solution:
Remove few screws from the bottom cover, to take off the plastic panel above keyboard (founded on YouTube). Disconnect screen cable (wider one on left hand side) from motherboard and you are DONE. Now when you boot up your laptop, it will work on external screen right from the beginning.