Using Target Display Mode with Big Sur & iMacs

I'm not sure if this will help anyone, but in regards to target display mode not working on Big Sur. 

I had a question on whether I could connect two late 2013 iMac 27" to my iMac 27" 2019 (using them as external monitors using target display mode). So I called apple tech support and was told I can't do this. (I'm not sure if I believe this, so I will be doing a test soon).

But during that call the adviser told me that I can no longer use target display mode with Big Sur. I hadn't actually upgraded to Big Sur, I was still using Catalina, because I was afraid of incompatibility and glitches with third party applications that I use. 

But I have a MacBook Pro 16" 2019 that is on Big Sur 11.3. So I decided to connect my iMac to my MacBook Pro using target display mode. And it worked absolutely fine. 

I decided to back up my iMac 27" to my external hard drive and then update it to Big Sur 11.3. (Just in case it doesn't work with target display mode and I need to revert back to Catalina).

After the update, I connected it to the 2013 iMac (which is still on Catalina) and target display mode worked. 

Here's the details of what I used.

My iMac 5k 27" 2019 is on Big Sur 11.3.

My iMac 4k 27" late 2013 is on Catalina.

I connect them using a genuine apple usb to thunderbolt adaptor.. and then plug in a  thunderbolt to thunderbolt cable, going from the iMac 2019 to the iMac 2013.

Hit command f2 on the iMac 2013's keyboard and all is well.

According to the apple advisers on the phone, it has nothing to do with the spec of the iMacs or MacBook, but instead Big Sur no longer supporting target display mode. But it works.

I can’t tell you why it works. Maybe the Big Sur 11.3 update allows target display mode. Who knows. 

I can’t tell you if it’s to do with it not working on any iMacs or MacBook Pros made after 2019… But all I know is that if you’re using the equipment (cables) I’m using and the same software combination, then it works for both MacBook Pro and iMacs.

Best thing to do is make a backup if in doubt and give it a test before updating to Big Sur 11.3. In future I will be backing up my iMac before making any operating software updates just to make sure that target display mode continues to work.

Hope this helps.

Side note: If anyone has managed to connect two iMac’s to another iMac as external monitors using display mode (So a total of three iMac screens) please let me know.