El Capitan doesn't boot with external cinema display

[ebbflow]

4k iMac wouldn't boot up after installing El Capitan. Tried recover and reinstall a few times, no luck. I tried pram restart. Only way to make the iMac start was in safe mode. Then I unplugged my display port from thunderbolt and voila it booted up no problem. I can connect the cinema display after starting the computer and it works fine. What's up? Video card?

Older 27" Apple Cinema Display and new 4k iMac

 

[iubhounds]

I have an older 27" Apple Cinema Display with a 5 year old iMac and have no issues with El Capitan and an external display. Sorry I can't help you. There have been some on this forum say there were issues with usb ports with El Capitan but I have all of my USB ports operating with no issues.

 

[verticaldubai]

4k iMac wouldn't boot up after installing El Capitan. Tried recover and reinstall a few times, no luck. I tried pram restart. Only way to make the iMac start was in safe mode. Then I unplugged my display port from thunderbolt and voila it booted up no problem. I can connect the cinema display after starting the computer and it works fine. What's up? Video card?

Older 27" Apple Cinema Display and new 4k iMac

I have the exact same issue with a Retina 5K 27" iMac (End 2015) and an external 24" LED Cinema Display. I can only boot without the Cinema Display plugged in, but once El Capitan (10.11) is booted I can plug in the external monitor and it works. Very strange. Anyone else have an idea what could be causing this?