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I'm using a 27" iMac in Target Display Mode, daisy-chained though a Thunderbolt display on my MacBook Pro. Both are running Yosemite.

MBP-->TBDisplay-->iMac (normal operation)

Typically I come into the office in the morning, connect my laptop to the Thunderbolt, then power on the iMac and switch to Target Display Mode. My laptop is now happily running two external displays:

desired setup

However, when I unplug my laptop to go to a meeting, the iMac grabs the Thunderbolt as a second display. I don't really care about this for now...

MBP-X-TBdisplay<--iMac (not a problem)

mbp disconnected

but when I plug the laptop back in and switch the iMac back to Target Display Mode, the Thunderbolt becomes the iMac's only display, and now the laptop only has a single external display.

MBP--xTBdisplay<==>iMac (problem with TBdisplay not showing MBP)

plugged back in

Is there any way of preventing the iMac from grabbing the Thunderbolt, or forcing it to relinquish it when I return, short of disconnecting and reconnecting the cable (really don't want to do this, the cable is flaky enough already) or rebooting the iMac? Even putting it to sleep doesn't seem to cut it.

  • There is the manual labor way :) use cmd+f2 to exit target display mode. You could write a script that is triggered when your system goes in to screen saver mode or sleep mode. – Ruskes Feb 5 '15 at 20:31
  • @Buscar웃 I wouldn't mind if that would work but unfortunately it doesn't. I want the laptop controlling both screens, but once the iMac grabs the thunderbolt display, it won't let go, regardless of cmd+f2. The iMac's own screen switches between laptop and iMac alright, but the thunderbolt display just sticks to the iMac until it's disconnected or the iMac is powered off. – CupawnTae Feb 5 '15 at 20:38
  • by not letting it go means what, what does it display if the master is off? are we talking about imac (the extended display) keyboard to use cmd+f2 – Ruskes Feb 5 '15 at 20:58
  • @Buscar웃 when I unplug the mbp, the iMac exits TDM, the video connection effectively reverses and now both displays show the iMac. When I plug the mbp back in, nothing changes automatically. Then I hit cmd+f2 on the iMac - the iMac's own display switches to tdm and shows the laptop, but the thunderbolt display still shows the iMac. If I power off the iMac at that point, the thunderbolt display switches back to showing the mbp. Then I can boot up the iMac, hit cmd+f2 and get back to where I want, i.e. all displays showing mbp. Wonder if I should draw pictures, it is a bit head-bending :) – CupawnTae Feb 5 '15 at 21:03
  • pretty good :) The last diagram doesn't quite capture the weirdness - the laptop's display signal is going through the the TBD to the iMac, and the iMac's video signal is going back to the TBD at the same time, but yeah that's basically it :) – CupawnTae Feb 5 '15 at 21:24
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A DisplayPort KVM would probably work. Have you tried using a DisplayPort cable instead of a thunderbolt one between the iMac and the Thunderbolt Display? It might be cheaper than a KVM which would guarantee you can switch inputs without the iMac taking control of the attached display.

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    Interesting - I've only tried a thunderbolt cable - don't think I have a mini DP to mini DP cable anywhere, but if I can find one I'll give it a go, thanks for the suggestion. – CupawnTae Jul 16 '16 at 16:14

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