A Thunderbolt Connectivity Conundrum

John Bertram wrote:

So this season, for the first time, I have a new MacBook Pro, which of course lacks the Mini-Display Port outlet of the older laptop. But everything I can find online suggests that all I would need to replicate the Old Configuration (which BTW DOES still work, since I also have the older MacBook Pro with me), would be a way to convert one of the new MBP’s USB-C outlets to a Thunderbolt-capable Mini Display Port outlet, just like the one already built in to the older MacBook Pro. So I ordered a not-inexpensive Apple-branded USB-C (Thunderbolt 3) to Thunderbolt 2 adapter, used it to connect my new MacBookPro to the iMac using the same Thunderbolt-enabled cable as before (which again DOES still work when connected to the older MacBook), and entered the Cmd+f2 on a keyboard connected to the iMac, directing it to switch into external display mode. 

And… nothing. No response from the iMac at all. Yet seconds later it DOES respond to the exact same keyboard command when connected to the OLDER laptop via the exact same Thunderbolt-enabled cable. 

Here's why: Apple added some restrictions to Target Display Mode.

The Mac that is going to run in Target Display Mode has to be running High Sierra or earlier. I don't think that would be the issue. A 2011 iMac can't run anything later than High Sierra even if you wanted it to do so.

The other machine now has to be a Mac introduced in 2019 or earlier, running Catalina or earlier. That is the killer. (And it's especially painful if you have a 27" 2009 or 2010 iMac, which didn't have a technical requirement for Thunderbolt input, and therefore would have been especially attractive for reuse by modern Macs and PCs.)

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

The Mystery: 

So is there anything I can I do to supply, from my new MacBookPro, that Thunderbolt-enabled cable (and through it the iMac display itself) the same signal it still happily detects when connected to the older MacBook Pro, the one with its own built-in MiniDisplayPort / Thunderbolt outlet? 

I don't think so. It appears to me that the issue is not with the Thunderbolt 3-to-2 adapter and Thunderbolt 1/2 cable – but with something that's changed in recent Macs (ones introduced after 2019) and recent versions of macOS (Big Sur and later).

You could probably set up IP-over-Thunderbolt between the new Mac and the old Mac using the Thunderbolt adapter and cable, and have it work. But if current versions of macOS refuse to recognize a Thunderbolt TDM iMac as a valid display, you're stuck.

John Bertram wrote:

I still think it's both surprising and very disappointing that Apple seems to have put less than zero effort into preserving the functionality of Target Disk Mode in the full iMac line, even promoting it as a major environmentally-friendly selling point. I have to think it could've been done, and I know it should've.

Disappointing – but not surprising. TDM is probably a feature that they threw in as long as they thought they could provide it for free – in terms of no per-unit hardware cost. With the 27" 5K screens (the screens which would be the most desirable to reuse), they may have faced a choice between adding / redesigning hardware, and dropping the feature.

What's more surprising are the new restrictions on "the other Mac". When I bought my Late 2009 iMac, the technical specifications talked about the machine being able to take Mini DisplayPort input, and did not say anything which led me to believe that there would be a "2019 or earlier" and "Catalina or earlier" restriction placed upon "the other Mac."

Just for reference, in my case the mid-2011 21.5" iMac I would have liked to keep using is running Sierra 10.12.6. (My new MacBookPro, as mentioned earlier, is running Ventura 13.4, and downgrading that is certainly not something I want to get into.)

You said that your new MacBook Pro is a M1 MacBook Pro, but if the Device Name ("MacBookPro18,4") in the snapshot means what I think it does, you actually have a 14" MacBook Pro with one of the high-end M1-family (M1 Pro, M1 Max) chips. Those came with Monterey, and you can't downgrade them to Big Sur. (The 13" M1 MacBook Pro came out earlier, so it can run Big Sur, but nothing lower.)

You certainly can't downgrade your Apple Silicon MBP to Catalina or earlier. None of the operating systems which support TDM shipped with ARM versions that would run on your processor's "bare metal".

What's especially frustrating is that when the two are connected, and I open the System Information window on the iMac and select the Thunderbolt pane (see screenshot), it does show that the iMac is at least "aware" of the new MacBook Pro's presence:

https://discussions.apple.com/content/attachment/3d6dc16b-c92f-4971-837b-47b09f075845

This is what keeps making me think there has to be some setting I've failed to activate, presumably on the new MacBookPro, that would remove whatever roadblock is currently in place. Something other than the Cmd+f2 combo that tells the iMac to become a Targeted Display.

Like I thought, the Thunderbolt connection is not the problem. There's every reason to think that the old iMac and the new MacBook Pro can exchange Thunderbolt protocol just fine. The issue is that the new system isn't willing to send DisplayPort video to the iMac, over Thunderbolt, for whatever reason.

(For instance, I still haven't gotten my brain around what "Thunderbolt Bridge" is for on the new laptop.)

I believe that it is Apple's current name for IP-over-Thunderbolt. (That is, using the Thunderbolt port on a Mac as a networking interface to connect to another Mac that's also set up to do IP-over-Thunderbolt. There was a similar IP-over-Firewire option long, long ago, when Firewire was fast enough relative to other inferfaces that it was attractive for some people to do local networking in this way.)

Use IP over Thunderbolt to connect Mac computers - Apple Support

But from what you write (and what the Help article you provided the link to spells out), it does sound like High Sierra & earlier won't be signing any mutually beneficial Trade Agreements with Ventura & Beyond anytime soon -- at least as far as the precious commodity of TargetDiskMode-ability is concerned. And that's a shame.

Target Display Mode. Target Disk Mode is a different feature. It lets one Mac boot up into a special mode that offers up its internal drive to the other Mac, as though it were a directly-attached disk.

Transfer files between two Mac computers using target disk mode - Apple Support

That said, thank you again for taking the time to help clarify this issue.

I don't know the answer, offhand, to the mirroring question.

It's very likely that you won't have any use for the Thunderbolt 3-to-2 adapter. There are specific circumstances (connecting a Thunderbolt 1/2 dock, connecting an Apple Thunderbolt-to-Firewire adapter) where you'd need it. But since TDM doesn't work with new Macs, you can scratch that potential use.

It really sounds like the only thing you'd be using it for is a fast data connection between your two Macs, for use with one of the screen reuse workarounds. If the screen reuse workarounds don't pan out, then you may as well return it. If you purchased it directly from Apple, in the U.S., then you have 14 days from the date of purchase to return it … otherwise, the return period (if any) depends on the retailer.

Returns & Refunds - Shopping Help - Apple

John Bertram wrote:

(And yes, I get that I could just toss the old iMac into the trash, shell out for a new external display with HDMI inputs, and feed THAT from the built-in HDMI outlet of my new MacBook Pro. But I’d still want to know why Apple was forcing me to do that, instead of their USB-C (Thunderbolt 3) to Thunderbolt 2 adapter making possible the same connectivity I can still achieve using the older MacBookPro.)

RE: Why they did what they did – you'd have to ask them.

With respect to the absence of Target Display Mode support from 27" 5K Retina iMacs, I believe that articles published around the time that the first 27" 5K Retina iMac came out suggested that the removal was due to technical reasons.

Yup, Servant of Cats is quite correct, TDM doesn’t work anymore—- about the only thing I’ve heard that might possibly work is “Duet Display/Luna Display”… link is here: https://www.duetdisplay.com/, it’s not free, but I don’t see a whole lot of other options if you want to keep using an older iMac as a display only

John B.

YES you can still use the old iMac as a display.

The way it worked before was through a feature from Apple - Target display mode, which newer Macs like your new MBP don’t support any longer.

The 2011/2 iMac has great 2k displays, and still usable with your mbp as a display, through third party software solutions.

For the purpose, the most popular apps use Airplay capabilities, such as Duet Display and AirServer. These are paid apps of a small cost. They both support extended display in addition to mirroring. The results are pretty good. I think they offer free trials.

Otherwise, you can set up free Remote Desktop apps so that your iMac captures and displays you mbp screen locally within the same network. The solutions are acceptable to some people, but won’t be as good as Target display or Airplay, since the content window will be transferred as pure image (no font rendering, for example). And, if you hope to extend the display, you need to get a dummy display dongle (EDID headless display adaptor, USB-C, or HDMI if your MacBook Pro still has it for -$10) which emulates a non-existing 2nd virtual display to your Mbp. The iMac can capture and display that virtual display.

The best Remote Desktop apps include TeamViewer and Anydesk.

Hope this helps.

There is no cable involved for third party Apps to work via Airplay or Remote Desktop. Only network access is required.

And, with your TB2 cable, along with a TB3/TB2 adaptor, you can set up a thunderbolt bridge between the two Macs. The new Mbp can use the storage of your iMac (along with all external disks and sd card) so that your iMac can also work as a big hub, in addition to acting as a display in parallel.