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iMac 27” mid 2011 target display mode not working

I am tearing my hair out trying to get my old iMac (27" mid 2011, High Sierra 10.13.6) to use as an external monitor with target display mode on my old MacBook Pro (13" 2012, Catalina 10.15.7). I'm using thunderbolt 2 cable to connect them. I've followed all the steps here Use your iMac as a display with target display mode – Apple Support (AU) and nothing happened. iMac won't display MacBook Pro. I’m using the original keyboard that came with the iMac and Apple thunderbolt 2 cable. Can't figure out why. Need help!!!


iMac 27″, macOS 10.13

Posted on Oct 28, 2024 3:07 PM

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Question marked as Top-ranking reply

Posted on Oct 30, 2024 6:32 AM

emmajunli wrote:

Thanks for the tip. Yup the target disk mode works. So we can rule out the possibility of a bad thunderbolt 2 cable connection. Too bad I’ve already ordered a new thunderbolt 2 cable… Plus the issue persists - iMac still won’t display MacBook Pro

I’m no hardware expert but I gather that a Thunderbolt cable carries 2 signals (besides power): data and video. There is still a chance that your current cable fails on the video signal (and not on the data signal) and that the replacement cable might work.

14 replies
Question marked as Top-ranking reply

Oct 30, 2024 6:32 AM in response to emmajunli

emmajunli wrote:

Thanks for the tip. Yup the target disk mode works. So we can rule out the possibility of a bad thunderbolt 2 cable connection. Too bad I’ve already ordered a new thunderbolt 2 cable… Plus the issue persists - iMac still won’t display MacBook Pro

I’m no hardware expert but I gather that a Thunderbolt cable carries 2 signals (besides power): data and video. There is still a chance that your current cable fails on the video signal (and not on the data signal) and that the replacement cable might work.

Oct 30, 2024 3:26 PM in response to emmajunli

It's a mystery to me why it ever works. 😉


There is still a possibility that the Thunderbolt cable you have is deficient in some way.

I'd try again with the new cable when it arrives and see if you get a different result.


I'm inclined to agree with @Keith Barkley. There's a reason Apple didn't stick with the feature beyond the 2014 hardware and macOS Catalina. It's so much easier to buy a monitor than to rig up an iMac computer to be a monitor - something it really wasn't designed to be.

Oct 30, 2024 3:45 PM in response to D.I. Johnson

D.I. Johnson wrote:

There's a reason Apple didn't stick with the feature beyond the 2014 hardware and macOS Catalina.


Apple dropped Target Display Mode when the first 27" 5K Retina iMac came out in Late 2014.


5K video has four times the bandwidth requirements for 2.5K video. That iMac had Thunderbolt 2 ports that had only half the bandwidth of Thunderbolt 3 ones, so there is a distinct possibility that it would have been necessary to split 5K video over two physical cables (rather than over two DisplayPort sessions within one cable).


Handling high-bandwidth video input in real time might have been too much to expect of the CPU – and the GPU might not have had provisions for offloading that work from the CPU.

Oct 29, 2024 7:02 AM in response to Servant of Cats

When I press Command F2, both screens will go black for a second, then back to normal. Yes I’m certain that I’m using the thunderbolt 2 cable. I also have a mini display port cable and I tried that as well - with that when I pressed Command F2, nothing happened at all, both screens wouldn’t even flick. I was thinking maybe my thunderbolt 2 cable is too old or something? Must be something really simple. I just can’t put my finger on it…

iMac 27” mid 2011 target display mode not working

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