Target Display Mode issues

I have two IMac 27" Both late 2012 models. Target Display Mode (TDM) has been working perfectly on both, these are connected with an apple supplied Thunderbolt cable

Imac 1 is my main computer, imac 2 is now used as the secondary display,

Imac 1 is running Catalina

Imac 2 was running High Sierra now upgraded to Catalina

When I upgraded the imac 2 to run Catalina this is when issue started - TDM can no longer be accessed with cmd + F2 or cmd+fn+F2 on iMac 2

When i use cmd + F2 on iMac 1 TDM works OK

It is imac 2 the one i want to continue to use as secondary display that seems to have the issue since upgrading.

I have tried rebooting both, moved the Thunderbolt Cable around with no luck!

I have previously upgraded the drive on imac 1 to a 2TB SSd drive with 8Gb RAM which is why this is used as primary machine, imac 2 has original hard drive with 4Gb Ram.

Can anybody assist?

iMac Line (2012 and Later)

Posted on Apr 6, 2020 12:43 AM

Top-ranking reply

CrashdoomMarshdevil

Level 1

4 points

Posted on Apr 7, 2020 11:54 AM

So, after talking with Apple Support, we've figured out the Thunderbolt cable is getting extremely hot at one end and is not recognized by either Mac (or a different Macbook we have here).

We used a smaller Thunderbolt cable to test if the iMacs were defective and TDM worked immediately, so the issue appears to have been the cable in my circumstance.

There's a slim chance that this was caused by the 10.15.4 update somehow frying the cable, but unless others have the same issue, that can't be assumed to be true.

TL;DR Thunderbolt cable was getting very hot at one end, replacing the cable fixed TDM

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Target Display Mode issues

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