External display not working after installing MacOs Catalina

"I tried the suggestion of purchasing CleanMyMac and it seams to have worked...for now"

Can you tell us what exactly did you do with this application to solve the display problem?

[I'm the one with the "ominous reply", by the way; well, many seasoned veterans share my advice about this program, as you can easilt attest by searching this forum]

I am glad that it worked for you.

I did not find anything disrespectful at all; I was genuinely curious to know.

The problem in a nutshel is that we do not know what this program considers "junk", and it has been known to delete "junk" that wasn't, leaving many a system with problems. Here's hoping yours keeps working fine.

Has anyone else tried contacting Apple support about this? Just spent over an hour on chat/phone with them and I can't get an answer out of them on whether they even know this is a problem. No one wants to admit that it's tied to Catalina...

To date, they've refused to admit the problem, because none of their support staff have a solution. I posted a frustrated complaint about it on Twitter and Apple support immediately asked me to message them so "they could help solve the problem." I did, explaining that I had already tried resetting PRAM, SMC, and every other thing suggested here and...they never responded. So much for being eager to help.

I have an LED Cinema Display circa 2010 (with the cables permanently built in--good thinking there, Apple!), and a 2012 iMac that's had a number of upgrades along the way, but still only has its original ports. Both machines were great and working perfectly right up until I made the boneheaded move of upgrading. I can only assume this is part of the endless push to get us to replace perfectly great equipment with new stuff? I don't know, but it's killing my productivity and I am SO frustrated.

I feel your pain ShandyGirl; but I wouldn’t put it off on Apple expecting folks to get new equipment. I just bought a brand new MacBook Pro on Black Friday. I had it upgraded to 64 GB of RAM, 2 TB HDD, the fastest processor they had, and the 8GB Video Ram on top of that. I could NOT have upgraded it to anything more than what I did. I brought it home, restored my previous data from before, upgraded to Catalina....then no longer had access to my external HDMI monitors. BTW...for anyone that read my previous posts...using CleanMyMac was NOT the silver bullet I was hoping it would be. It worked 4 days for me...but now my monitors aren’t working again.

This is ********. Lost 30 minutes today AGAIN with restarting to safe mode etc. And I disconnect my macbook 3-4 times a day. Piece worth 3000$ works worse than the olders windows notebook I have. I wont stop myself going with reviews where I can, cause it must be fixed asap. Catalina is not tested ********.

test tips: left top: charger, left bottom: usb-c - display port to DELL, top right usb-c to display port to DELL, top bottom usb-c to usb-3 adapter with ethernet cable and three usb 3 to other things like extarnal mac keyboard cabled and audio DAC and one free. Ofc I've tried to mix them, but that config is best - another funny thing with 3k$ piece, why order matter?

Same. My iMac has no USB-C, and has never needed it until this horrible update. What's really crazy is that it knows the second monitor is there. If I do a screenshot, two images are produced, including what is presumably on the second screen (just the desktop background), but the display itself is off. This seems like something that would be an easy fix if Apple and ANY of its developers would just acknowledge the problem and do an update.

Same issue here - worked in December of last year - went on holiday came back, no connectivity with external monitor using USBC. Replaced with a brand new cable and still nothing. Tried all hints and advise here but still no external screen connectivity.

This is with a MacBook Pro (13-inch, 2017, Four Thunderbolt 3 Ports) connecting to an Acer H277HKsmipuz 27 inch UHD Monitor.

Something has definitely changed over the last month.

I don't understand. You say you use the HD and daisy chain the display off of it. When you disconnect the HD, OF COURSE the displays go black... what am I missing?

It would make much more sense to do it the other way around, I suppose, since you are unlikely to disconnect the display - this is a Mac Mini, after all. Then disconnecting the HD should have no effect on the display.

But if I understand correctly, you may be forced to do it this way since the Mini has only one Thunderbolt port.

In that case, does the hard drive have a different connection like USB3? If so, I suggest you take the HD off of the Thunderbolt chain. You are not gaining any speed from using Thunderbolt, as the hard drive itself is the limitation, not the bus.

Alas, I've tried this numerous times over the past few weeks, with no result. My computer knows the monitor is there (and will, in fact, take a screenshot of it), but the monitor itself is blank. It's been three months and crickets from Apple on this issue, which is pretty disheartening.

Thanks @LIONSxLEADER Your update got my Dell U3014 back up and running at proper 2560x1600 in RGB mode. Only alteration to your process that I had to perform was to mount and access the /Volumes/"SSD serial"/ volume and not its ' - Data' partner volume, since Catalina on APFS splits OS and data.

@Luis -

I think you are misunderstanding what I have said. My Thunderbolt display is connected via thunderbolt cable into my Mac Mini, the LED Cinema Display is plugged into my HD via thunderbolt cable then the HD is connected to the back of my Thunderbolt Display. If I unplug the HD, it shouldn't affect the Mac Mini + Thunderbolt Display at all.... it never did previous to this update, but now if I unplug it it causes the Thunderbolt Display to go completely blank and then I have to Force Shut Down and start over.

Unfortunately this "daisy chain" setup was the only way I could use the LED Cinema as a 2nd monitor due to what Apple did when they discontinued it for the Thunderbolt. If it were as simple as connecting the LED Cinema to the Thunderbolt I could've avoided the expense of buying the LaCie d2 Thunderbolt altogether.

Even after speaking with a Senior Advisor at Apple, she had no clue as to why this issue was happening. We did everything and it is still not resolved.

Ok, I see. Does the hard drive have an alternative connectivity like USB3?

If so, then I would connect it using USB3. No loss of performance (USB3 is plenty fast enough that the hard drive itself is the limiting factor), and it would eliminate the need to daisy chain it - unless, wait, are you saying that the Cinema Display doesn’t work if daisy-chained directly off the Thunderbolt display? If that is the case, then it is a shame.

Luis -

Funny thing is that the HD does have a USB3 port, but the Thunderbolt Display only has USB2 Ports. Back when this all happened I was livid because Apple had almost rendered my 27" LED Cinema Display obsolete when the Thunderbolt came out. Luckily, LaCie came out with their d2 and I was able to continue using it. The issue was the switch from "Mini Display Port" to "Thunderbolt" - they're not compatible.

I am having this same exact problem, the very second after installing Catalina.

It worked fine before, now I have to wait 30 minutes or more for the extra monitor to show up.

Why is there no real fix to this issue from Apple when so many are having a problem?

I've tried everything in this thread. Nothing works!