I have a Mid-2015 15" Retina Macbook Pro running the latest macOS Sierra version with the AMD Radeon R9 M370X dedicated GPU and I have been trying to get it hooked up to my Crossover 324K 4K monitor without success.

The symptom I am experiencing is that I will plug a the DisplayPort end of my DP to mDP cable into the DisplayPort connector of the monitor and the Mini DisplayPort end into any of the two Thunderbolt 2 ports on the Macbook. The screen resolution scaling switched back to default on the macbook and I see the external monitor light up and sometimes display a picture for a fraction of a second and then it goes right to no signal and the resolution changes back to what it was on the macbook as if no external display was plugged in.

The monitor has an option in the settings to toggle between DisplayPort 1.1 and 1.2 modes. I can get 4k at 30 hertz to run if I set the option to DP 1.1 but the monitor doesn't work at all if I set it to DP 1.2.

Additionally, everything works flawless on windows with the latest AMD drivers so I suspect something about mac os is to blame. I can have the monitor running at 4k, 60 hertz and DisplayPort 1.2 on windows 10 just fine... The last oddity I have discovered is that I can boot up the Macbook into osx with the lid closed and the monitor plugged in (in DP 1.2 mode) up until the login screen which renders without retina scaling. As soon as I type in my password to decrypt the FileVault disk and login, the display goes blank and switched back to the internal monitor so I'm basically stumped...

  • Have you tried a different HDMI cable? - I have experienced in the past that some HDMI cables do not work correctly, even though their specs say they should. For some of them it seems random when they do and do not work. Might be worth a try if you have several cables lying around. – jksoegaard Feb 15 '17 at 6:43
  • also consider that you have to look for the right settings at both the Macbook and the display regarding 30 Hz (single stream) and 60 Hz (multi stream). Unfortunately Apple did not consider to be all set for future use. – pallox Feb 15 '17 at 11:05
  • @jksoegaard I am using a DisplayPort cable with no adapters or HDMI conversion. The graphics card in this laptop doesn't even support HDMI 2.0 which would be required to run the display at 4K 60hz. – Tristan Feb 15 '17 at 15:28
  • @palloxI was able to open the system profiler and have it refresh the connected hardware during the split-second where the monitor does show a picture before disconnecting and it shows that it is running at 3840x2160, 60hz so I believe the OS does negotiate the MST link but for some reason it just immediately disconnects. – Tristan Feb 15 '17 at 15:30
  • You could try SwitchResX and see if some option allows 4k 60Hz to run properly. – JMY1000 Feb 15 '17 at 19:19

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