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I'm using a MacBook Pro 15 inch retina (mid 2012) and I'm using a 27 inch iMac as an external display (connected using Thunderbolt) and another 23 inch display connected using HDMI.

System Specs

I'm looking to replace the 23 inch display with a larger one and looking at my options. I've read a few posts on here and the general gist of what I'm reading is that it's possible to hook up a 4K display via HDMI running at 3840 x 2160 @ 30 Hz. My first question is if this would still apply when I keep the 27 inch iMac as a second external display. Does anyone have experience with this? It's working fine with my current setup, but the 23 inch display is only running at a 1920x1080 resolution.

Update 9/6 So I decided to give it a shot and went for a Samsung U32H850UMN display. I'm connecting it to the HDMI port of my Macbook and at the same time the iMac is still connected in TDM using the thunderbolt port. The Samsung display is running at 3840 x 2160 @ 30 Hz (as expected). The Macbook is running in clamshell mode. I'll keep an eye on the Macbook's temperature to make sure it doesn't get too hot. If it does I will just switch to just the Samsung monitor as my main display.

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Officially these MBPs can support up to one HDMI display (at 1920x1080 pixels) while simultaneously connected to a Thunderbolt display (at 2560x1600 pixels), OR being connected to two Thunderbolt displays (both at 2560x1600 pixels).

However, unofficially you can exceed this. I'm not aware of anyone who has done exactly what you're wanting to do, but I have come across the following scenarios with other users:

  • connecting two 27" Thunderbolt displays each at 2560x1600 pixels while also using the built-in display, with a 23% increase in the MBP's fan speed, but with no long-term damage/problems.
  • connecting three external displays while in clamshell mode. However, this can result in the MBP's fans spinning at more than double their usual speed when watching videos etc. Not something I would recommend.
  • using a single Thunderbolt display at higher resolutions (e.g. 3440x1440) with the MBP in clamshell mode without any problems whatsoever.
  • using two 27" iMacs in Target Display Mode (one using a Thunderbolt connection and the other a Thunderbolt/DisplayPort connection) without any problems (both in and out of clamshell mode).
  • using a single Thunderbolt display at higher resolutions (e.g. 3440x1440) while also using the built-in display. This seems to work okay, although the fans will spin about 30% faster. This is probably the limit of how much I'd recommend pushing things.

It's not clear from your question whether you were wanting to use the MBP in clamshell mode, but the info above may give you enough to help you decide whether what you want to do is worth the risk.

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  • Thanks for this insightful answer, that's really helpful! I was hoping to be able to use the laptop screen as well, but if that would cause the GPU to produce too much heat I'm open to using it in clamshell mode. What's interesting though is that using two 27" iMacs in TDM would work fine out of clamshell mode. I guess I'll just have to experiment to see what works best without pushing the temp too much. Any recommendations for specific display models?
    – Ruben
    Commented Aug 31, 2017 at 17:22
  • Actually, I've had to edit out the second part of your question as it is really a separate question altogether. :) I don't really have any recommendations, but in terms of the user who's using the two 27" iMacs in TDM, neither is connected via HDMI, so that may be a factor worth considering since the Samsung you're looking at only offers HDMI, while the other two also offer DisplayPort inputs. You probably really need to check out some reviews and the official support pages for the ASUS PB287Q and ViewSonic VP2780-4K UHD displays.
    – Monomeeth ♦
    Commented Aug 31, 2017 at 23:18
  • Okay fair enough. :) I'll update this thread as soon as I get the monitor, hopefully it will help some other people as well.
    – Ruben
    Commented Aug 31, 2017 at 23:42

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