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I've got a MacBook Pro 13" with Lion plugged into an external VGA monitor. Often, when I unplug its video dongle deal, the MacBook doesn't realize the other monitor is gone. It continues to believe it's the Secondary Monitor (which I can confirm by right-clicking the desktop and selecting "Change Desktop Background...").

There's no menu bar, so I can't get to my Display icon to Detect Displays. No, I don't want the MacBook screen to be the primary when I'm on the external.

Plugging the DisplayPort dongle back in does nothing. It won't even re-display on the external monitor it was hooked up to before, so no mirroring kludge if I forget. To this point, I hard power down and restart.

Any idea how to tell the MacBook there's no external monitor and that it's the primary now?

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Command + Brightness Up is the keyboard shortcut for Detect Displays, but that option is gone now in ML. –  vcsjones Jul 26 '12 at 14:14
@vcsjones he said he is on Lion, which should clear his problem! –  Philippe Gilbert Jul 27 '12 at 0:18
@vcsjones -- might want to turn that into an answer. Seems to do the trick in 10.7. –  ruffin Aug 1 '12 at 18:32
@ruffin Added as answer. –  vcsjones Aug 1 '12 at 18:43

2 Answers 2

up vote 14 down vote accepted

Command + Brightness Up is the keyboard shortcut for Detect Displays in Lion.

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On my MBP with ML, It turns out that Option + Brightness-Up turns on detect displays. –  user26557 Aug 1 '12 at 23:02
Jumping into this conversation 2.5 years later (HELLO PEOPLE OF THE FUTURE) to say that this also works a treat in Yosemite. –  Scottie Feb 17 at 15:57

Another way to do this is by going to Displays in System Preferences and then holding down the ⌥ Option key and the Gather Windows button will turn into a button that says Detect Displays.

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