Q: Multiple Displays have problems with El Capitan

@ Waveform2 - sorry you're having so much difficulty with your iMac and external display.  I have recently upgraded all my iMacs and 1 MacBook to El Capitan.  One of the iMacs (27" 2009) had the issue of non-response to HDMI, but later resolved itself after disconnecting everything and slowly adding back all the other peripherals.

I have 2 other iMacs, one 27" 2010, and one 21" 2011 - both are running an external HDMI monitor at 1080p using El Capitan without issues.

If you can, shutdown the computer and disconnect everything from the back except for your keyboard or mouse if they are hard wired.  Restart your iMac and immediately hold down the shift key to boot into 'safe mode.'  Let the computer complete it's start-up routine completely, then try plugging in your Mini Display HMDI adapter and see if it works.  If so, restart the iMac (normal - not safe mode) again without any other things connected.  If the monitor is still working ok, then add the other peripherals - starting with hard drives first.

This procedure has worked for me when one of my iMacs showed the issue you're describing. 

Dan

Thanks i'll give it a go and report back.

That didnt work buddy but thanks for the idea, still the same as if theres nothing there i even tried it into my PC monitor which has HDMI as well.

Just adding my voice of frustration. I need two external monitors to work (Bloomberg terminals) and the upgrade now has forced me to use my mac air to do work while I run my macbook pro for Bloomberg.  Oh and forget about sharing docs now across the platform. How many months does this have to go on?

Two monitors, one USB and one firewire.  Firewire works great regardless of which monitor so obviously it isn't the monitor but the USB.  The port works well with other peripherals.  Did the reset, even wiped clean the BOTH macs and no good.  This is unacceptable to have a performance issue like this go on this long.

I agree mate and especially the price we've paid for our hardware.

It has to be software because its not just old macs or new its them all my displayport wont work and im guessing your on a newer platform? Cries software to me actually i know its software because it worked before i upgraded!

Any news if 11.1.4 fixes the multiple monitor problem? (I have a new mac pro, 2 x D700, 32GB RAM, 2 x DELL2715H - connected via thunderbolt, 32TB Thunderbolt RAID, Blackmagic thunderbolt to SDI converter). I tried 11.1.3 and that didn't work - only one monitor would show unless I unplugged everything else. So I'm stuck on 10.10.5 cos that works.

TIA

Ziggy

My situation is I have a new Mid-2015 MBP 15" and two external 22" ASUS displays connected by Thunderbolt to HDMI cables. I've only had this for a few days so I'm still trying to characterize this issue but for me, if I boot up the MBP by itself (neither Thunderbolt cable plugged in) then plug them in, the displays work fine (so far). If I put the computer to sleep (such as when I walk away till it times out or when I return to work in the morning) and try to revive the displays, the laptop and one of the displays lights up and stays fine. The 2nd ASUS randomly blacks out for 1-5 seconds and then turns on again. It's annoying as heck!

Rebooting the laptop with the cables attached doesn't fix the problem.

Just installed the latest El Capitan OS X update 10.11.4  and it doesn't fix the problem in my case.

When I unplugged the thunderbolt cables and rebooted, then plugged them back in, so far it's working fine. No issues.

Yeah it just dosent work 100% for me pathetic this is STILL an issue!

Nice shiny new apple stuff but don't worry about your existing customers.

I'm trying another theory now as there are others in our company using almost the same configuration (MBP + 2 displays) and who aren't having or at least haven't reported any issues yet like mine.  They use one Thunderbold and the HDMI port on the MBP. So I got an HDMI-HDMI cable and plugged that in for my "bad" display. So far it's working flawlessly. But the acid test is when I return after a solid sleep such as overnight. I'll report back if that is my "fix".

Another option is that one of my Thunderbold-HDMI cables is bad. I will verify that next if this HDMI-HDMI cable proves to be working solid. I have a Thunderbolt-HDMI dongle that I can re-use the same HDMI cable to the display and also allows using the 2nd Thunderbolt port to see if the cable was bad or not. Hopefully what I just said is not confusing.

It appears that one of the HDMI-HDMI cables (of the two I ordered) was bad. I have returned it to Amazon. I'm now using a Thunderbolt-DVI cable to my 2nd monitor (it's what I had on hand) and all is well. No issues using my two ASUS 22" displays and my MBP laptop screen open as a third smaller screen.

I may have been able to get a replacement HDMI-HDMI cable but opted to go with the proven cable I had on hand that works fine.

Similar problem here.  Upgraded to el crapitan and now my hdmi adapter is supposedly not on a list of approved devices?  am i supposed to spend money on an adapter because of a software update?  It is definitely not the cable or adapter itself, which has worked flawlessly up until now.  Thanks Apple.

In case you are using USB display adapter you should download display drivers on the following link: http://www.displaylink.com/downloads/osx

I had the same issue after I install El Capitan on my Imac. I had one screen on thunderbolt and another screen on USB port. The screen on thunderbolt worked but the one on USB port didn't work. After I install USB display drivers now both works.

Thanks for this tip Millport.

I have an old 20" cinema display and a new 21" imac. Connection is via an adapter to thunderbolt. The cinema was working fine for a while; once or twice the system did not recognize it, but rebooting fixed this. New development as of last week was total non-recognition, eg in the system profile it showed nothing connected to thunderbolt. I tried a bunch of fixes gathered from here and there, but this one was the one that finally worked.

Cheers for that!! Worked a treat. Much obliged.

I was having the same problem - slow startup and only one monitor working when I upgraded to El Capitan.  Disconnected everything except my main monitor an keyboard and immediately noticed the much faster startup. I added second monitor and restarted.  Still fast, both monitors worked. Added thunderbolt doc with other peripherals connected, still worked.  Finally added an external usb drive and it went back to slow startup and one monitor.  Unplugged it and everything worked again.  Yeah, I finally have my monitors back. I even added a third monitor in my HDMI port and it still works.  Love being back at my 20 second startup vs minute plus.  Thanks BereaVideo!!

Mac Pro Quad 3.7 Ghz, 16 GB, 512 GB