External monitor problem iMac 2017 High Sierra.

When you've started your Macbook, try to wait until you plug in your adapter. 10 seconds or so should do it. Then plug in your adapter AND wait another 10 seconds or so until you plug in your VGA, DVI, HDMI or whatnot.

I am also experiencing issues after upgrading to High Sierra.

I had 2 external monitors running at 4k and I am experiencing two issues.

1. One monitor outputs a blue screen. Sometimes a unplug/plug in helps sometimes not.

2. My external sound card drops the sound randomly when the monitors are plugged in.

Seems there is something with this that Apple broke that was working in Sierra.

Will update if I find any changes.

Matthew

Same issue here, wish I had never done the upgrade to High Sierra! I really need the second screen and now it isn't available!!I have a 2016 MacBook Pro and a late 2013 IMac same problem with both.... Please fix ASAP Apple, I am a dedicated Apple user for all platforms of technology sooner than later would be best!

I have this issue too on my late 2013 MBP after upgrading to High Sierra.

The issue is only with the 2nd thunderbolt port. The 1st one works fine.

If I plug the monitor into the 2nd port it detects that there's something there and just tries to initialize it constantly and never succeeding (literally the screen turns on, detects no video source and turns itself off... then mbp turns it back on, and the loop repeats itself). I haven't tried the HDMI port, mostly because the monitor doesn't support 4K properly on HDMI.

I also have an ethernet dongle that only works in the 1st thunderbolt port and not the 2nd.

It's not just a display issue.

I am having the same issue. I just updated my iMac to OSX High Sierra and my second display stopped working. I can move the mouse to it but no content is shown. It was working fine under sierra. Very annoying, breaks up my workflow. 😟

Can report the same here from London. Upgrade to High Sierra yesterday and now only one of the two external monitors works. I have a small professional home studio so it feels as though someone's pinched my mixing desk!

Same here. I am able to get the external monitor to come back on by unplugging the Apple Mini DisplayPort to VGA adapter then plugging it back in to my external Thunderbolt 3 dock. After that, if the Mac goes to sleep and I try to wake it, there is no response unless I unplug and plug it back in. Occasionally the resolution will become stretched out. Unplugging and plugging back in solves that problem, but this is not ideal. At least I don’t need to restart the Mac each time.

My late 2012 MacBook Pro won't boot if the Apple Thunderbolt display is plugged in (I have to plug it in once the laptop's booted. Then once working with it I have all kinds of graphics issues–frequent corruptions, inability to display some video. The laptop on its own is working fine (well, fine-ish–I still get a bunch of permanent freezes that I never had pre High Sierra).

so i have two ASUS monitors, and mid-2012 MacBook in the middle - left monitor is OWC usb to dvi adapter. right is thunderbolt to HDMI... Sierra, all worked great. then after updating to High Sierra, i'm with the same drama as those here. only the laptop and one monitor would work. or if both connected, the left and right would work but laptop would not. reached out to the guys at OWC where i purchased the usb to dvi to HDMI adapter and advised to update the drivers for the usb adapter, then do the 'resets' after a shutdown...still didnt work fully both the externals would show but the laptop would not.

it was suggested i drop resolution on the laptop itself, (but monitors are still the same). so my laptop now is 1280x800, and the two monitors are still 1080p as they were pre-upgrade.

so i'm at least back to where i was before, but not 100%.

Maybe this helps someone else, until apple fixes the real problem - have a great day!

I had the same issue after updated to the lastest macOS High Sierra 10.13 Supplemental Update (Oct 8, 2017) and I've solved it reseting the SMC on my MacPro using the link below:

http://www.idownloadblog.com/2016/02/03/how-to-reset-smc-mac/

If you run Time Machine backups just go back the point before installing High Sierra. Until apple admit/fix the problem then I see little point in upgrading. That's the only thing that worked for me done everything SMC etc.. and a fresh reinstall of High Sierra, none of which made any difference!!! Certainly would not spend any more money for something that appears to be Apple's fault.

I have to agree. A new monitor will not help this issue. This is clearly something Apple has to work out. I have one of the latest model MacBooks and a 2 new monitors, so buying a new monitor is not going to change anything. There is something that will get fixed. Use a Time Machine back up. I went back to Sierra and everything is working as expected! Don’t waist any money untill this issue is resolved.

Hey :-) While I don't want to deny you a nice new monitor if you want one, but my three not-so-new ones are now suddenly back up and on. I'm not sure how I fixed it. I tried using Apple TV for a bit, that didn't really work, but what was curious was when I did get the Apple TV to link up to my 2011 iMac, it showed a desktop image I have used for ages, so not sure wherever one of the caches plays a part here..? All 3 working fin now though!

No, I didn't have a working Time Machine backup from just before the upgrade (something went wrong with the hard drive I was using), so had to follow instructions on how to rollback and reinstall from a backup made after the upgrade, but the rollback instructions left me with no working OS so I had to do a full fresh install of High Sierra from an erased internal hard drive.

Teach me to forget to test the backup.......

OK... There is no official timeline on this but...

Apple Seeds Third macOS High Sierra 10.13.1 Beta to Developers | MacRumors Forums

Lets hope this answers all our issues...

macOS High Sierra 10.13.1 appears to focus on bug fixes, performance improvements, security enhancements, and other under-the-hood changes, but it also introduces a range of new Unicode 10 emoji...

One thing is for sure the emoji team is not slacking!

Matthew