External monitor problem iMac 2017 High Sierra.

It's really annoying. I have the same issue like many others. The thing is, if a cable works in a Dell with a NEC monitor in my case, why when I switch to my Mac pro the same cable HDMI to VGA doesn't work... It should be a software problem... I don't want to buy many cables to see if at least one may will work... Totally upset... Thanks to every contribution, but it seems that Apple doesn't care anymore...

I've given up on trying to use any thunderbolt to HDMI adapters. Nothing I try will work. Therefore, I have decided to purchase a good HDMI splitter (a 4 x port one that provides an ON/OFF switch for each port).

Hope that you finally get your connection working again).

Bye

Having this very issue with the recent upgrade done today where I updated to High Sierra 10.13.2, I was running the older version of HS before this.. My external Dell monitor was connected via the MBP's DVI port to the Dell's HDMI. As soon as I did the update the external screen is no longer working with my MPB late 2011.

I did the drivers update. I know the trick of disconnecting the cord to the monitor and then reconnecting. If I keep connecting and reconnecting the DVI adapter eventually the system will recognize the external monitor is connected but no image will be displayed on that monitor.

My DVI to HDMI adapter is an apple adapter and I've had it for maybe 5-6yrs so I am going to try to see if a new adapter might do the trick, but as it has been pointed out before, it is rather suspect that this happened right after an update.

For anyone with an older monitor that uses a DVI connection and needs Thunderbolt 3 (USB-C) to connect to a 2017 iMac – I spent several weeks researching, finally ordered what I thought was the perfect adapter, and it was a complete fail. The iMac didn't even recognize that another display was connected – zapping the NVRAM, resetting the SMC, nothing helped at all. Did some more research and found this cable, and it worked like magic the second I plugged it in!

https://www.amazon.com/gp/product/B0761LK6XF/ref=oh_aui_detailpage_o01_s00?ie=UT F8&psc=1

I installed High Sierra on my Macbook Pro and then my mini display-port to HDMI stopped working. I tried every reset and different HDMI cables, nothing worked. Then I found the problem. When installing major updates on Mac's if you leave devices plugged in, as the computer restarts several times during install power surges through your ports can knock out your external devices. Well that's what happen, it broke my mini displayport adapter. after using a new one, problem solved. High Sierra or Sierra can knockout Thunderbolt docks, display adapters or anything connected to your computer. So next time, unplug everything connected to your Mac before updating.

I was running two displays on my MacBook Air with no issues.... I upgraded to High Sierra and only one works... It's an Apple issue and they need to get it fixed!

Hi. I have the same issue with an external monitor. It worked fine (Sierra) before I installed High Sierra (Macbook Pro - Dell P2011H)

try resetting your SMC

[instructions copied from http://osxdaily.com/2010/03/24/when-and-how-to-reset-your-mac-system-management- controller-smc/]

SMC reset fixed it for me: How to reset the System Management Controller (SMC) on your Mac - Apple Support

I have the same exact problem. Apple! Please fix this! Productivity is being greatly affected!

Same here and with 12 of my clients machines. All of them are now on one monitor when they dock at work.

This is such bull sh!($. They need to fix the issue now. It's obviously a well know issue at this point.

High Sierra 10.13 Supplemental Update fixed it for me probably. Both external displays are now working (DP & HDMI).

Thanks, tried this (in various different orders and also waiting 30 seconds) but it made no difference......