Using an Apple Silicon Computer in Target Disk Mode

This morning, I needed to transfer several large files (video files created from Tivo recordings) from my new AS MacBookPro to my 2020 Intel iMac. I decided to do that by attaching the MBP to the iMac using the Thunderbolt 3 cable that came with my CalDigit Thunderbolt hub and attaching the MBP to iMac by starting the MBP in Target Disk Mode. Traditionally that has been done by starting the computer while holding the ‘T’ key down on the keyboard. However, that doesn’t work with an Apple Silicon Mac.

The procedure, discussed for computers running Monterey is described by Apple Support here.

After connecting the computer, you put the AS Mac in Recovery Mode by holding down the power key until startup options are loaded. You then click Options Icon and Continue and then supply a password for an administrator id. Up to this point, this is what you would do for a recovery. Now, however, from the menu bar that has appeared, you click Utilities → Share Disk. The disk(s). Select the one to be shared and click ‘Start Sharing’.

The fun is just beginning. Rather than the shared drive of the AS Mac appearing as an external drive on the other Mac, it appears as a location on the Network either via the Go->Network menu or under the Locations->Network item in the sidebar of a Finder window. Then find the file(s) to be transferred and copy or move them. The speed makes it worth it.


This is because essentially this is no longer TDM as we know it (where the Mac behaves like a TB-SATA bridge), but rather the Mac running a NAS service out of recovery mode. Essentially the Mac is now running an SMB server out of 1 True Recovery’s macOS. This is very different from the old TDM, where there was no macOS running at all and instead the SATA drive was just being shared over TB (or FW before that) as if the entire Mac were nothing but a bridge dongle (like this one). Of course to the user it should in principle make no difference, with a few clicks (instead of a boot and hold T) you should get fast access to the Mac’s internal drive. But what’s happening underneath is indeed quite different.

As so often, Howard Oakley has some interesting background on this new SMB target disk mode.


This implies that you might do just as well leaving macOS running normally on the AS Mac and use normal SMB file sharing over Thunderbolt for this kind of file transfer. If the AS Mac isn’t doing much else at the time, I would expect performance to be similar.

Also, given that the AS Mac is running an SMB server, I wonder how the performance would compare against doing this via a direct Ethernet connection (via GigE or 10GigE). I would expect GigE to be significantly slower, but 10G might be similar, depending on how well macOS is able to saturate that 40G Thunderbolt link during file transfer.

Very interesting Alan and Simon.

I would probably not have thought to do this for the use case you outline and would have proceeded along David’s line.

I’ve only ever used Target Disk mode when things were bad, the Mac unreliable. I wonder what the break point here is in this new scenario. How will this perform when a Mac won’t boot for example.

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It shouldn’t really make a difference actually. SMB TDM needs a functioning disk so it can load 1TR. And without a functioning disk, even the old TDM would not have worked. OTOH, a functioning disk should be good enough because all you need is 1TR which is sealed and separate from your boot container and its data. If your boot macOS is hosed, it’s 1TR you’d be booting into. And if even that were hosed, you’d go into internet recovery and restore that first. You could perhaps argue now you also need a functioning network so if your NIC where somehow damaged, this would break. But that’s no different from the old TDM where if your FW or TB controller somehow were damaged, it would’t have worked either. Looks to me like Apple thought this through. It might sound like more overhead and more things that could break, but in reality it’s probably a wash. If TDM worked in the old days, chances are SMB TDM will work now.

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