Does Catalina support Target Display mode?

To spare anyone tedious scrolling, here's the answer:

The simple answer is NO.

Target display Mode only works on High Sierra or Lower (although in my case, it worked on Mojave and vanished after I updated to Catalina)

The article is: https://support.apple.com/en-au/HT204592 and the following is stated:

"Target display mode requires macOS High Sierra or earlier. It does not work with macOS Mojave or later".

Furthermore the article states: This article has been archived and is no longer updated by Apple.

No. Below is my config.

Used as a display:

• iMac 27-inch, Mid 2011
• macOS High Sierra 10.13.6

The host computer:

• iMac 21.5-inch, Retina 4K 2017
• macOS Catalina 10.15.1
• With clean install to an older version than upgrade it to Catalina

Connection:

• Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter
• Apple Thunderbolt Cable (2.0 m) - White

Just to be absolutely clear: the iMac that is to be used in Target Display Mode must be running High Sierra or earlier.

The other mac that will use the iMac’s display CAN use Catalina. That is the configuration that I’ve been using with no issues whatsoever.

Unfortunately it appears that an iMac running Catalina can no longer function in TDM.

A Catalina mac can use the display an iMac with an older OS that is set to TDM (something I do daily with my Catalina MBP to my High Sierra iMac).

I don’t know if this is lack of support in Catalina itself or if installing Catalina updates firmware and that kills support. If it is the former it may be possible to revert to Mojave or dual boot for the times you want to use TDM. If it is the firmware, then that is probably impossible to get it back.

"Make sure that your iMac is using macOS High Sierra 10.13.6 or earlier."

(From https://support.apple.com/en-ca/HT204592)

The problem is not with the iOS version in the MBP, but with the version in the iMac. I updated first the MBP, and I did not have any problem; then I updated the iMac and the display mode stop working. Apple support says that the version in the iMac has to be High Sierra or earlier, "You can't use target display mode with later versions of macOS". Do you know why they discontinued this mode? It is very useful, we do not have space to have 3 screens in our desks.

So...

I got MBP 2017 (10.15.2) and iMac 21" 2013 (10.14.6)

Im using USB-C to Thunderbolt 2 connector and Thunderbolt 2 to Thunderbolt 2 cable

TDM works for me. But I cant share sound to iMac

I have a 2011 iMac and I use it daily in Target Display Mode with my 2014 MBP. It continues to work now that the MBP is running Catalina. (The iMac remains at High Sierra which the furthest it can go).

I don’t know if this is dependent on models, or if the tb3-tb2 adapter causes any difficulty. Just wanted to confirm that the 2011 iMac works for TDM.

I am using iMac 27-inch, Mid 2011 (macOS 10.13.6) as a display and the host machine is iMac 21.5-inch, 2017. I am using the "Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter" and the "Thunderbolt Cable (2m)" bought from Apple Store.

It works when I am using macOS Mojave 10.14.6 but not works after upgrading to macOS Catalina. I already tried many methods, but still not works.

I would try resetting NVRAM and SMC and see if that helps.

FWIW, I am using a 2011 iMac (running High Sierra 10.13.6) as a display for my 2014 MBP (running Catalina), and it works just as it did in Mojave.

The most obvious difference is the TB3->TB2 adapter (my mac still has TB 2 so I only use the Thunderbolt cable), but that should still work.

Make a backup or two, clone your boot storage to a scratch test environment, boot that, and upgrade and test that.

And is it wise to perform a production upgrade to Catalina, absent some Catalina-only requirement? Without testing?

Then let us know which (presumably) iMac you have, and whether it works with Target Display Mode. Or what (else) breaks.

We’re all still sorting out what works, and what doesn’t, and what works differently, based on the churn here in ASC.

~60 different questions either new or new replies in the past ~hour, mostly Catalina problem reports.

I waited a while to upgrade to Catalina, I now find that Target display mode is not working - My 2017 iMac running v10.15 no longer displays to my mid 2011 iMac running 10.13.6. When I cmd f2 to launch the display of my 2017 to 2011 nothing happens. I have rebooted both, unplugged the thunderbolt cable (inc Adaptor - all Apple) still no luck.

Any ideas?

FWIW. I have been following this post and found that what users have experienced vs. what Apple says supports TDM seems to be in conflict. Apple is stating that to use TDM, a device needs to be running OS X 10.6.1 (Snow Leopard) or later. It doesn't mention anything about issues using this mode with the macOS series. Of course, anything is possible.

The other thing that appears to conflict is which exact iMac models are supported. The article states only three specific model can be used as displays. Yet, a number of replies in this post infer that a Mid 2011 model works, which is not one of those Apple listed. Even the "Questions for" section of this support article conflicts with the article's requirements as well. Go figure!

Ref: Use your iMac as a display with Target Display Mode - Apple Support

All I know is that I could use TDM on a daily basis without any issues, I then upgrade to Catalina after reading that other users have had no or little issues, so took the plunge to find that my workflow is now considerably suffering due to TDM no longer working as it should. I suspect that the launch of SideCar has had a random impact on TDM. Maybe an update will fix this error?