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I have two 27" iMac connected via a Thunderbolt cable. I press Command+F2, the target computer joyfully becomes a second screen for the primary iMac. Joy of joys!

I come back in the morning, or after lunch, and I press a key on my computer. The display lights up, everything is where it should be.

Unfortunately, the display computer stays off. I haven't found a way to bring it back on, other than pressing the power button, waiting for it a little (this disables target mode), then pressing Command+F2 again. This, of course, re-enables target mode.

Any idea what I could be doing differently and better, for a wholesome dual-27" experience?

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You can bring it back on by going to System Preferences / Displays and choose Detect Displays. It can take a couple of minutes to get it back, though.

Apple suggests a SMC reset when a computer that supports target display mode does not switch into or out of the target display mode as expected.

To do so, just:

  1. Shutdown the computer which is not functioning as expected.
  2. Unplug the computer's power cord.
  3. Wait fifteen seconds.
  4. Attach the computer's power cord.
  5. Wait five seconds, then press the power button to turn on the computer.
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I'll have to try that; thanks. – Aldric Sep 12 '12 at 14:42

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