I was wondering if anybody knew if there's like a physical switch or sensor that let's the MacBook Pro know when its in clamshell mode. I know you need an external keyboard and/or mouse and close the lid as soon as you boot up. I assume there's something that lets the machine know that the lid is closed. The reason I ask is because since my MacBook Pro fell and broke its screen, I'm planning on buying an external display to use. Currently, I have the MBP hooked up to my TV via VGA and keeping the lid closed with only a mouse hooked up. I'm planning on going headless, and I don't want the MBP to think its lid is closed and display in spanning mode. I already opened up the machine to fix the casing and to look at the display, but I didn't notice anything that would tell the machine that the lid is closed. Or better yet, is there a way to force the MacBook Pro to use the external display and display in mirror mode only at boot up?