Using iMac mid 2011 as a second monitor for my macbook pro

den.thed wrote:

I know that the TDM article is hard to fully comprehend.

Also the 2018 MacBook Air may not work, if it has been upgraded to Mojave or Catalina.

Indeed, It has us confused!

It is the iMac used as a display that is constrained to no higher OS than 10.13, NOT the SOURCE computer. Catalina still supports TDM on pre-2020 Macs.

Recent success report with steps for TDM and Catalina here.

https://discussions.apple.com/thread/251899135?answerId=253627813022&page=1

Carefully study > Use your iMac as a display with target display mode - Apple Support

You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display. Learn how to identify your iMac model.

• 27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable to connect your iMac to a Mini DisplayPort or Thunderbolt or Thunderbolt 2 port on the other Mac.
• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display. It does not work with macOS Mojave or later.

In a nut shell, it will not work because your 2020 MacBook Pro does not support TDM.

I'm trying to do the same thing, and while I haven't been successful yet, I do know what the problem is. Your old iMac does not have a Mini Display port, it has a Thunderbolt Port. The Mini DP plug looks identical to and does fit in the thunderbolt port, but it will not function as a monitor connection. I've been searching all day for a thunderbolt 1 or 2 to thunderbolt 3 cable, but they don't seem to exist. The best I could find is a thunderbolt 2 to thunderbolt 3 adapter which costs about $45 and then you still have to buy a thunderbolt 2 cable if you don't already have one (like I don't) which is apparently another $35! Good luck.

Connect the Thunderbolt 3 plug of the Apple Thunderbolt 3 adapter

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple

to the 2018 MacBook Air and a Thunderbolt cable

Apple Thunderbolt Cable (0.5 m) - White - Apple

from the Adapter to the 2011 iMac.

I know that the TDM article is hard to fully comprehend.

The problem is that Apple has changed ports 3 times in the last 8 years and you must use the exact adapter and cable that are specified in the article.

Also the 2018 MacBook Air may not work, if it has been upgraded to Mojave or Catalina.

ellamont83 wrote:

However, I tried to link the iMac with my 2018 Macbook air and I got the same result- it didn't do anything. Do you think it could be to do with the cable I'm using?

Thanks!

Yes..likely the cabling and not the OS version. People are successfully using TDM from 2019 Macs runnig Catalina.

Reports indicate that best chances are to run a Thunderbolt 3 cable from the SOURCE computer (your 2018 MBA) and put the adapter on the iMac.