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I use an iMac with Target Display Mode from a Macbook Pro. Sometimes when I enable Target Display Mode, the iMac screen stays black, even if my Macbook Pro's display suggests that another monitor has been plugged in (apps move away from the screen).

How can I fix this?

  • Posted my answer in a similar thread here. Hope this helps. – debuggr Jun 8 at 17:05
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On the Macbook, change the rotation on the iMac display to 90°, then revert it back (or let it revert back after the timeout) to Standard. That seems to snap it out of its zero-brightness funk more or less every time much more frequently than other display changes.[1]

(Of course, you can't do this in clamshell mode, you need some functioning display to use System Preferences→Displays on.)

[1]: (I've seen it fail once or twice, but it is the most reliable workaround I've found yet.)

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On closer inspection, it looks like the display is not actually uniformly black. This led me to conclude that perhaps the backlight was just not turned on properly, and led me to discover the following workaround:

  1. Log into the iMac
  2. Change the brightness to a different level (e.g. up or down by one). The level seems to need to be different from the one previously used.
  3. Enable Target Display Mode again, by pressing Command+F2
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    This doesn't seem to work super reliably for me. For while it seemed like it did, but I'm trying to get TDM to kick in right now and it's just black no matter what buttons I press. – Glyph Oct 12 at 22:54
  • @Glyph just coming across this issue now and so far the only fix I've found is to log out of the account on the iMac so I'm at the login screen before hitting command+F2. Would be interested to hear if the same works for you – CupawnTae Oct 13 at 16:23
  • I've got the same issue these days, and it no longer works for me either. The way it failed almost seemed like it was a slow hardware death of sorts rather than a software issue, although the iMac still works fine when not in Target Display Mode. – m01 Oct 14 at 9:40
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    @CupawnTae Logging out on the iMac doesn't work at all; it just doesn't recognize ⌘-F2, won't go into TDM at all on the login screen. However, I have found another workaround which seems reliable: on the macbook, change the rotation to 90°, then revert it back to Standard. That seems to snap it out of its zero-brightness funk more or less every time. (Of course, you can't do this in clamshell mode, you need some functioning display to use System Preferences→Displays on.) – Glyph Oct 15 at 4:27
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    @m01 Done. Words words words because SO has a minimum comment length. Words. – Glyph Nov 5 at 22:14
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Resolved. Had to uncheck 'Automatically adjust brightness' on the iMac's display control.

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