I have a MBA connecting via HDMI to Samsung Monitor. Problem is desktop background for Samsung monitor is not showing. There is only grey desktop and doesn't have any changes even I try to change the image of the desktop at System Preferences. Changes for the MBA desktop is ok. Is that a mountain lion bug?

  • I have a somewhat similar setup (MBP and LG external) and haven't seen this problem when switching to Mountain Lion. When you go to the "Desktop & Screen Saver" preferences, does it show you two dialogs...one for the main screen and one for the "Secondary Desktop"? – Phillip Mills Aug 5 '12 at 15:11
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    Grey screen...are you sure you're not running an app in full screen mode? – Samantha Catania Aug 5 '12 at 15:25

Restarting the Dock seems to work for me, however I don't know what is causing this grey desktop background.

To restart the Dock, type this into Terminal and press enter:

killall Dock
  • This fixed it for me. Thanks. Pretty strange Mountain Lion bug. – gak Nov 25 '12 at 22:27
  • Worked instantly, great tip. Have been wondering about this for months. – Matt Gardner Jan 13 '13 at 22:15

OS X allows you to select different images for each display. When you open the Desktop section in Desktop & Screen Saver Preferences, you should get one dialog on each screen for choosing a desktop image. Each dialog controls only the image for that screen, so make sure you're setting the image you want on both screens.

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    This setting doesn't seem to save if I disconnect and reconnect the second screen (like when I leave from work and take the laptop home but plug it back in tomorrow). – powerj1984 Nov 15 '12 at 14:40

I also experienced this problem and found this workaround:

  • Open System Preferences -> Displays -> Arrangements
  • switch on "Mirror Displays" and switch it off again

This is no solution for the problem of course, but at least the wallpaper is showing again without rebooting...

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