Target Display Mode issues

Question

Level 1

11 points

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

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Answer 1

Top-ranking reply

CrashdoomMarshdevil

Level 1

4 points

Apr 29, 2020 6:46 AM in response to PeterW.Bax

Yes, under Mac OS Catalina 10.15.4, Target Display Mode works between two 27-inch, Late 2012 iMacs, per this official Apple guide: https://support.apple.com/en-gb/guide/mac-help/mh30822/mac.

Only older iMacs prior to 2014 support Target Display Mode. My issue was a faulty Thunderbolt cable, replacing that got it working again.

Attached a screenshot with my username in it, and your post, to confirm:

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Answer 2

Apr 7, 2020 9:06 AM in response to dialabrain

This is completely incorrect. It was working until the latest update 10.15.4 upon which my 2012 iMac stopped allowing itself to enter TDM with my other 2012 iMac.

Also, the article you linked to is archived and completely not related. Here's the updated article that says this should be possible: https://support.apple.com/en-gb/guide/mac-help/mh30822/mac

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Answer 3

Apr 7, 2020 10:05 AM in response to dialabrain

No, it's still not working. As of updating to 10.15.4 today, TDM completely stopped working on either 2012 iMac. Feels like this is a bug with 10.15.4, so I'm trying to restore using a Time Machine backup to 10.15.3.

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Answer 4

Apr 7, 2020 11:54 AM in response to CrashdoomMarshdevil

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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Answer 5

Apr 7, 2020 2:17 PM in response to dialabrain

Yes, running Catalina 10.15.4 on both iMacs, using a smaller Thunderbolt cable appeared to work.

The old cable doesn't seem to work at all any more. I'm getting shipped a replacement cable under warranty so I anticipate that will rectify the issue.

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Answer 6

EVways Author

Level 1

11 points

Apr 8, 2020 2:40 AM in response to dialabrain

Ok, as an update, i have now installed 10.10.5 on display imac, when i check System report>hardware>thunderbolt i can see cable is connected, i can also see this connection on my main Imac running 10.15.3 as well -so cable appears to connect at both ends. cmd+F2 still inoperative! any assistance on my next move would be appreciated? Would use shorter cable but 0.5m will not reach between two imacs side by side! I would rather not spend £35.00 on a new cable if this is possibly not the issue!

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Answer 7

Apr 9, 2020 7:16 AM in response to dialabrain

Just got the new cable in and it worked perfectly after plugging it in. Both 2012 iMacs on Catalina seem to be able to use TDM on each other.

The new cable also doesn't heat up on either connector, so I can only assume something caused the cable to short and break. If it's heating up and not being recognized, contact Apple Support to ask for a replacement cable.

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Answer 8

dialabrain

Level 10

130,717 points

Apr 6, 2020 12:47 AM in response to EVways

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

From: Use your iMac as a display with target display mode - Apple Support

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Answer 9

EVways Author

Level 1

11 points

Apr 6, 2020 12:49 AM in response to dialabrain

Hi - thanks for your reply it is however working when used on my main Imac (iMac1) which is running Catalina! so i don't understand this?

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Answer 10

dialabrain

Level 10

130,717 points

Apr 6, 2020 12:51 AM in response to EVways

The host Mac can be running Catalina, not the target. That's my understanding.

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Answer 11

dialabrain

Level 10

130,717 points

Apr 7, 2020 9:38 AM in response to CrashdoomMarshdevil

Then I guess you have it working.

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Answer 12

EVways Author

Level 1

11 points

Apr 7, 2020 9:48 AM in response to dialabrain

Not at all, I have used Apple support, on their advice also I erased hard drive of display iMac to remove Catalina then have re installed Mountain Lion, TDM is still not working!