Target Display Mode Hell (2011+ iMac, Blackmagic) *OR* egpu : '15 rMBP -> iMac
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Target Display Mode Hell (2011+ iMac, Blackmagic) *OR* egpu : '15 rMBP -> iMac  


New Member
Joined: 2 months ago

I have done a lot of searching but have yet to find a definitive answer. I'm pretty sure neither of the below options will work but can't hurt to ask.

The setup I am working with is the following: 

  • Mid-2011 iMac with TB1 port (not Displayport)
  • Mid-2015 rMBP pro 15" w/ Radeon R9 370x


I've kept the iMac because as I can use it as a second monitor with my work MBP (2013 13-inch) via TB2 -> TB3 adaptor. Works fine for this and I still use it for web browsing, etc. Have resisted a second monitor due to space constraints. I had initially been waiting on the M1 iMac to replace it and use something like Luna Display as a second monitor but the M1 didn't turn out to be specced as I had hoped for the price (plus gen 1 issues, etc.). 

An eGPU would be a nice stopgap for a few years. Here is what I am wondering, out of mostly stubbornness:

  • Has anyone ever successfully gotten a Target Display Mode on a 2011+ iMac to work over a TB connection output from eGPU - presumably has to be Blackmagic. I found this thread on Apple's support site where Blackmagic support suggested this would work, but I have my doubts and have not found a report where this worked. TDM needs to receive an OSX signal from the host machine and I am not fully confident it would carry that signal all the way through from the MBP to GPU to iMac


  • Is it even possible to run this setup: egpu <-> 2015 rMBP -> iMac. Basically taking the TB signal back through the MBP TB port 1, then outputting to the iMac through MBP TB port 2. Yes it's TB2, so the limitations would be probably too much, but I have also heard that I may be stuck with the dGPU outputting to the iMac no matter what so it wouldn't even work at all. 


I don't think either of these are great solutions, but would be great to know from anyone's experience for sure before looking at other options. For what it's worth I'd be using this in OSX anyway, hence the TDM stuff. 

This topic was modified 2 months ago

To do: Create my signature with system and expected eGPU configuration information to give context to my posts. I have no builds.


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Noble Member
Joined: 4 years ago

I've never seen ioreg from a Mac that is connected to an iMac for Thunderbolt TDM so I don't know that the GPU is even involved. It could be a virtual display like DisplayLink.

I believe you can connect MBP -> eGPU -> iMac but that doesn't mean the iMac display is controlled by the GPU of the eGPU (or even the iGPU).

Mac mini (2018), Mac Pro (Early 2008), GA-Z170X-Gaming 7, Sapphire Pulse Radeon RX 580 8GB GDDR5, Radeon Pro W5700, Sonnet Echo Express III-D, Trebleet Thunderbolt 3 to NVMe M.2 case