MacBooks started in Target disk mode not showing in Finder on iMac

[mtfitzgerald]

I have read at least 20 posts on this in various forums without finding a solution. Here is the situation and what I have tried so far:

• Core issue is a 2019 16" MacBook Pro that has had a coffee spill. Touch bar does not work, and it reports a Panic crash after 180 seconds of not receiving input from the Touch Bar (dfrd read). It does boot correctly to that point and is fully usable while alive for three minutes.
• It stays on indefinitely when I start it up in Target Disk mode, no matter which of the two ways I use to start the Target disk mode.
• I don't have a Time Machine backup for the Pro and want to recover the files before sending the MacBook Pro for repair. I am using my Late 2015 27" iMac for this. It is on the latest Monterey version. The iMac has Thunderbolt 2 ports and the MacBook Pro has Thunderbolt 3 ports. I have therefore bought the Apple T3 to T2 adaptor and an Apple T2 cable this morning to connect the two computers. The MB Pro is also on Monterey, though not the latest version as it won't stay on long enough to update it.
• When connected, which I do before booting the MacBook, it does not show up in Finder or the desktop on the iMac. I have also looked in the network locations, and at the Thunderbolt Bridge configuration. This shows no IP address on the iMac end. I don't know whether or not that matters. Renewing the DHCP lease does nothing. Turning off Wifi changes nothing.
• Suspecting that the problem might be with the MB Pro, I booted my wife's 2021 MacBook Air and connected it to the iMac the same way, just to see what would happen. Nothing. The MacBook Air does not show up on the iMac when booted in Target mode. Both are on the same current Monterey version.
• No encryption and no firmware password anywhere.
• To check that the problem is not with the iMac, I started the MB Pro in Target Disk mode while connected to the MacBook Air using a USB-C cable. It does not show up on the MacBook air. Note that the Thunderbolt port shows as existing when I look at the system report in About This Mac. However, it does not show anything as connected to the ports when either MacBook is connected.
• The MacBooks started up in Target mode are not visible in Disk Utility either, so they are not just sitting there unmounted.
• The MB Pro stays on long enough to complete the First Aid test in Disk Utility, without finding a problem. I have also booted it in diagnostic mode and the online hardware diagnostic does not find any problems. I suspect that only the touch bar has shorted, but have no way of proving that.
• And finally, when both MacBooks are in Target Disk mode, they do indeed show the two expected icons moving about their screens, suggesting they have booted correctly. If either actually connected at any point, I would expect one of the two icons (Firewire and Thunderbolt) to disappear.

My feeling is that I must be missing something really basic about Target mode. I just can't work out what.

I have tried things mentioned in various forums, not just here. I am reasonably strong technically, so can probably handle complex instruction. All suggestions welcome.

Maurice

 

[Cory Cooper]

Hello and welcome.

Due to the coffee spill, that could have damaged the MBP ports as well, which may prevent it from connecting in target disk mode. However, that doesn't explain why the MacBook Air doesn't connect to the iMac.

I wonder if it may be the cable you are using. You would need an actual Thunderbolt cable, not just a USB-C cable, like the charging cable that comes with the MacBook Air and MacBook Pro. Thunderbolt cables with the USB-C style connector have the Thunderbolt icon on the ends. Standard USB-C cables do not.

Target Disk Mode Not Working on Your Mac? Here’s How to Fix It
HOW CAN I TELL IF A CABLE IS USB-C OR THUNDERBOLT?

C

 

[mtfitzgerald]

Hello and welcome.

Due to the coffee spill, that could have damaged the MBP ports as well, which may prevent it from connecting in target disk mode. However, that doesn't explain why the MacBook Air doesn't connect to the iMac.

I wonder if it may be the cable you are using. You would need an actual Thunderbolt cable, not just a USB-C cable, like the charging cable that comes with the MacBook Air and MacBook Pro. Thunderbolt cables with the USB-C style connector have the Thunderbolt icon on the ends. Standard USB-C cables do not.

Target Disk Mode Not Working on Your Mac? Here’s How to Fix It
HOW CAN I TELL IF A CABLE IS USB-C OR THUNDERBOLT?

C

That is worth a try. As the Apple quote for the parts alone was $1,400, I took it to a local repair shop and asked them to clean off all the coffee stains. That made no difference. The next step they proposed is to open it again, disconnect the keyboard/touchbar assembly, and connect another keyboard. Assuming the second keyboard has a touchbar, that will at least answer the question of whether the touchbar has shorted and is causing the crashes. I will only have the answer to that on Tuesday evening. I will try connecting the Air to the Pro with the charger cable after that.

A minor but unlikely thought I had was this. Is there any form of account check when you start up in Target mode? By this I mean, does the same user have to have an account on both Macs? I have not seen that in any of the instructions I have read. It does seem like a security issue if not.

 

[Cory Cooper]

OK, let us know how that goes.

Target disk mode bypasses loading macOS and doesn't use any of the users on the Mac. It turns the Mac into and external drive.

C

 

[mtfitzgerald]

OK, I have the MB back from an unsuccessful simple repair attempt. It was simply cleaned from top to bottom, inside and out. No change to the symptoms. So, I tried connecting the MB Pro to the MB Air using the TB charging cable this time, in Target Disk mode. I confirm that the MB Air does not see the Pro in Finder, and it does not appear in the Disk Utility. The TB ports on the Pro all work. All four can read a USB memory stick that I insert.