MacOS Catalina Mac Mini 2018 LG 27UK850

Same here with an LG 32UD99-W.AUS monitor and a 2018 Mac mini. I have to plug into my TV first, and then plug into the monitor. If it starts with the monitor connected, the monitor is blank and gets no signal. They better fix this quick...

I have a similiar issue after upgrading my 2018 Mac Mini to Catalina. I use a 4k Samsung TV as a monitor, and after the update, when I turn on the Mac, the TV reports no HDMI signal, and nothing is displayed. I logged in with Jump desktop, and the Mac. has booted OK.

I found that if I shut teh MAC down from the Jump Desktop session, when I restart the MAC the TV displays fine. This seems to be a repeatable workaround.

I have a case open with Apple Support and sent detailed debug info to them.

***Update*** This solution seems to be only temporary. After two days, the HDMI port stopped working and the Thunderbolt/DisplayPort connection became active. I did not shutdown or reboot, it seems to have switched over on its own.

Seems that we will just have to hope that this gets fixed in the next Catalina release.

The workaround of using a USB-C to HDMI cable seems to be a good possibility, but that cable costs about $20. Not too expensive, but it will become a wasted expense once Apple fixes this problem.

An update to my earlier post. Have found that Catalina with the Supplimentary update is workable with an eGPU with an RX580 card if you do the following:

• Turn everything off
• Turn on Mac Mini (2018)
• Wait for it to properly start up (say 1 min)
• Turn on eGPU and Monitor (connected via TB3 from the Mini and DisplayPort to the Monitor)

Every time you will get the log-on screen and you are good to go.

If you need to restart during a session, then turn off the eGPU, wait for the Mac Mini to restart and then power up the eGPU. Am hoping the 10.15.1 update will fix this, but if it doesn't the good news is that it supposedly supports RX 5700XT cards, so can finally upgrade to something with a bit more grunt =)

New Update (Wed 30 Oct 2019)

Just installed the 10.15.1 Catalina update. Ufnfortunately the eGPU RX580 bug has NOT been fixed. Still have to cycle the power on the eGPU to get it to work with the Mac mini. =/

What is the solution? Should I get a new Monitor?

Yes. I am not sure what to do.

Updated 2018 mac mini from Mojave to Catalina and Catalina supplemental and same failure, no longer working with hdmi into Dell U2718Q 27" 4k monitor.

My Solution

I ran into the same issue. I have a Samsung U28E590D 4K monitor that had no issues with Mojave, using HDMI to HDMI. With Catalina I got this cryptic "Not Optimum Mode" screen. Tried using a HDMI to USB-C adapter and no display at all. Yesterday I got a DisplayPort to USB-C cable, no issues plugging it in while my Mac Mini was up and running. This morning when I started up my monitor appeared to get a signal but no Apple logo, etc. I had to plug in my Acer non-4K monitor HDMI to HDMI to log in (which is what I have had to do since I did the upgrade).

Apple is working on the issue, and will probably fix in an upcomming release. In the menatime, this cable worked for me, as it bypassed the Mac's HDMI port:

https://www.amazon.com/gp/product/B075V5JK36

I also have a Samsung U28E590D with a Mac Mini 2018 and the same issue (come on Apple!) I discovered that I can just give it a few minutes to get the password screen up (even though I can only see the "Not Optimum Mode" error. And typing in the password as normal, then enter obviously, and then after a few seconds, the HDMI input kicks in and starts working. Not ideal sure, but a lot less hassle than messing about with more monitors.

Similar problem. Update took down the HDMI output from my Mac Mini 2018. Could not connect a monitor to the computer via HDMI or eGPU (RX 580).

Search online (eg. "macrumors.com") and there are a *lot* of people complaining about this. Huge bug and amazing that Apple/ Developers missed it.

Best solution so far is to take video from one of the USB-C ports, either via a USB-C to HDMI cable or (the option I chose) USB-C to DisplayPort.

Luckily after the October 15 "supplimental update" the eGPU option appears to be working again (ASUS Studio-XS + RX580). Am hoping things are properly fixed for OS 10.15.1 =)

Same problem with Mac Mini to LG monitor via HDMI. Worked fine with Mojave. Broke with Catalina. It's really tricky, but here's a way I got the HDMI connection to work:

1. Connect Mac Mini to the LG display using a Thunderbolt-to-DisplayPort cable so that you can get Mac Mini to boot and see it on the LG.
2. Shutdown the Mac Mini via the Apple menu.
3. Unplug the Displayport cable from both the Mac and the LG.
4. Connect Mac Mini to LG using your HDMI-to-HDMI cable.
5. Switch your LG monitor video input selection to the HDMI (*** important, it will not work if you don't do this step***)
6. Press power switch on Mac Mini to boot up.

When I did this the first time, the HDMI connection worked normally for a couple days. Then it stopped working again and I had to repeat this procedure. Clearly a fix from Apple is needed, but for now this procedure might keep you going for a day or two at a time.

The HDMI to USB-C did not resolve my issue. The only way I can get my Mac Mini to start is to plug in the non-4K monitor, then switch to the 4K monitor.