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I have a Macbook Pro with Mac OS X 10.7, and an external display screen attached to my Macbook, which is working fine.
Sometimes, I have to switch the second display to a projector (because I want to give a presentation).

Those settings (e.g. arrangement, resolution) are different from my external display screen. In 90% of the cases, Mac OS X knows which arrangement/resolution I want, depending on the type of external screen; in 10% of the cases, it's mixing up the configurations and I have to manually edit the configuration again.

Can I see somewhere what configuration is attached to a certain display?
Why does Mac OS X mix the configuration settings between my external display screen and my projector?

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Look in the ColorSync in your Utility folder, there the profiles for the displays are saved, and it shows which one is used for what. – Buscar웃 Sep 5 '14 at 4:21
It depends. More details are really needed regarding your device. Some models simply have more than one GPU. – tjt263 Oct 6 '15 at 13:50
Is the external display on when you connect it? – Thorbjørn Ravn Andersen Feb 27 at 0:09

1) You should be able to see these configurations in System Preferences > Display. I believe you go to to Layout (or could be called mirroring).

2) OS X displays two Display panels so you know which panel is attached to which screen.

If the Display options don't allow you to pick a resolution, change the Resolution from "Best for XX display", that will allow you to pick a supported, lower, resolution.

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1) I can see the current configuration, but not other configurations (when I attach my external LCD screen, I cannot see the configuration of my projector) 2) yes, but it's showing an incorrect display – Jochen Hebbrecht Aug 31 '12 at 6:29
So the additional panel says something along the lines of Generic LCD? – squarefrog Aug 31 '12 at 7:43
@ squarefrog: what do you mean with your last question? – Jochen Hebbrecht Sep 10 '12 at 15:18
In the additional display config panel, it should say the name of the display at the top. I'm asking if it says Generic LCD or something similar? – squarefrog Sep 11 '12 at 10:37
@ squarefrog: it was saying the name of the projector, but it should have said the name of my LCD screen – Jochen Hebbrecht Sep 11 '12 at 13:51

You might want to investigate a third-party solution.

I would try SwitchResX (Macupdate link) as it claims to be able to save the resolution better than OS X can on it's own.

I do not own this software.

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