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I have had a dual-screen setup (27'' iMac + 27'' thunderbolt display) for a while now; having recently acquired a MDP->HDMI adapter I'm struggling with mirroring the secondary display onto my TV.

TV is the highlighted one

The screenshot above shows an arrangement where the desktop is extended across all three screens. Now, if I select "Mirror Displays" then I get all of the displays mirrored i.e. same image on my iMac, thunderbolt display and the TV, which is not what I'm after.

Is there some obvious setting I'm missing? Can this be accomplished without third-party software?

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up vote 2 down vote accepted

Hold down your option key and drag the TV onto the secondary display.

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I don't have three displays to check this out with, but if this actually works it's the best hidden OS X tip ever. – CajunLuke Oct 25 '12 at 16:52
This, in fact, allowed to mirror the display, however I was limited by the resolution of the TV (i.e. I end up having my 27'' monitor running at 1080p as well if I want mirroring). I'll ask another question unless you're familiar with this issue. – o.v. Oct 26 '12 at 6:57
The resolution list is created by using only those that are supported by both displays when in mirroring mode. – Lance Helsten Oct 26 '12 at 21:35

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