Você pode tentar isso, a Dica 1 funcionou para mim, mas você deve fechar a tampa durante a inicialização / reinicialização antes que o logotipo apareça. Quando a tela de login aparecer, você poderá abrir a tampa e a tela deverá estar desligada. Pode verificar com os botões de controle de brilho.
Esta não é uma solução ideal, pois fechar / abrir a tampa toda vez pode ser bastante incômodo, mas funciona (mais informações abaixo). Uma opção da Apple para desligar isso com monitores externos conectados seria ideal.
Link: link
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.