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?
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:
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.
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...