After a recent OS update, Clamshell mode quit working. I couldn't find any Apple documentation
that would help. So many people were stabbing around in the dark, until I finally found the following information
that is lacking from any Apple documentation that I've been able to find.
Here's what I found to be the most helpful:
Forcing a MacBook to use the External Monitor:
I tried about everything: Rebooting, switching off completely, removing and inserting the Thunderbolt->HDMI adapter etc.
The only thing that finally worked was the following:
1. Take it out of the dock and connect to Power and HDMI to the TV.
Shut down the Mac. Turn it on and close the lid. It will now boot up on the external screen.
Log onto the Mac, and put it in sleep mode (Option-Command-Eject).
Put it back into the dock and wake it up with the spacebar.
It should be OK now... until the next update...
Note: One indication that you’re going to have extreme trouble with this is when (occasionally and without reason) the Mirroring Options icon shows up on the top icon bar.
Other useful commands:
“Command + F1” puts PC in Two-Screen MIRROR mode.
If it’s REALLY being stubborn, do the following (Clamshell Must be open
to see the arrangement tab.):
In System Preferences > Display on the MacBook screen there should be an Arrangement tab when you have the MacBook hooked up to the TV and both screens working. When you click the Arrangement tab do you see two monitors side by side? One of them will have a Menu Bar at the top. Just click on the Menu Bar and drag it to the second monitor (Yes, I know, it’s hard to grab ahold of it...) That will make your EXTERNAL monitor your main screen.
You can now use your MacBook in Clamshell Mode with a wired or Bluetooth keyboard and mouse. http://support.apple.com/kb/HT3131 When you disconnect from the TV, make sure it’s asleep (Option+Command+Eject buttons) or your Menu Bar will automatically change back to the MacBook.