Esta página sugere uma solução alternativa:
- Set the display mode to mirroring (use the F7 button).
- If you haven't done so already, attach the external screen, keyboard and mouse.
- Press a key on the keyboard (like Shift) and click the mouse once.
- Close the lid and wait for the Macbook to go to sleep.
- While sleeping, click the external mouse once, the Macbook will wake up.
If all goes well, the external screen will display the Dock and menu-bar after a few seconds. As soon as you see this, the Apple logo on the Macbook shouldn't light up, open the lid of the Macbook. The Macbook won't go to sleep, won't power up its built-in screen and you can now use clamshell mode in lid-open mode.
EDIT: Veja os comentários sobre este post para experiências de kch com este método.