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I started bringing my 2011 MacBook Air to work and hook it up to an external monitor for dual display setup. But when I unplug the display, the desktop wallpapers on the built-in LCD disappears and just show a solid light gray desktop. I have to log out and log back in to get the desktop wallpapers to show again. Is there a way to fix this?

Many apps also still behave as they were on the external monitor. Do I have to use "gather windows" every time I disconnect the external monitor?

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I noticed that once I upgraded my late-2011 MBP to 10.8.1 I had to reset all my background preferences as well, external 27" at work - both displays set to randomly display from a custom folder every hour. I forgot exactly what I did but eventually the settings did 'stick'; try setting the preferences the way you want without the external monitor, log out, connect the monitor, log back in, and try setting them again. Rather, rinse, repeat.. – da4 Sep 12 '12 at 13:18

I also observed the problem after upgrading my MacBook Air (2011) to Mountain Lion. Not only unplugging the external display triggers the gray background, but also connecting to a display.

For the gray background problem, I use a quick-and-dirty fix: restart the Dock. Invoke the following command in the terminal: $ killall Dock

The Dock will be restarted after it is killed, and the wallpaper will appear again.

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After some exprimenting, it seems that disconnecting the external display first while the macbook is still open fixes the issue. The problem only arise if I close the lid before unplugging the external display.

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Another workaround is to open Terminal and type "killall Dock" at the prompt.

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