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Possible to boot from a MBP (in Target Disk Mode) on a 2020 iMac?

ultrafiel80

macrumors newbie
Original poster
Feb 20, 2019
12
42
Does anyone know if you can put a 2017 MacBook Pro into Target Disk Mode then connect it via Thunderbolt 3 to a new 2020 iMac and boot off of that disk (essentially using the MBP as an external drive to boot from)? I'm trying to look for an option to use my new 2020 iMac screen with my work MBP. I just upgraded from a 2011 iMac where I had Target Display Mode, so it's unfortunate that I can't do that anymore (but I knew that when I purchase it). I don't want to drop $40 on a Thunderbolt 3 cable if this isn't possible, but it seems like it should be. Thanks.
 

VolceOntra

macrumors 6502
Apr 25, 2007
311
77
I haven’t been successful in doing so.

Apple support told me that I’ll be able to do so in Big Sur but I don’t think they know what they are talking about. I think they just wanted to shut me up for a while.
 
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ultrafiel80

macrumors newbie
Original poster
Feb 20, 2019
12
42
I haven’t been successful in doing so.

Apple support told me that I’ll be able to do so in Big Sur but I don’t think they know what they are talking about. I think they just wanted to shut me up for a while.

You know, I was able to get this working last night. Wasn't as straightforward as it should have been, but I booted up. When I get a bit more time I need to write up everything that I did. I did have to boot into Recovery mode as change some options so that I could boot from an external disk (as well as other versions of the OS. I forget the name of that utility, but I'll look into it again.
 
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Fishrrman

macrumors Core
Feb 20, 2009
20,451
7,256
The utility you "changed" settings in, is called "startup security".

One of the first things I did with my 2018 Mini was to boot to recovery, and then DISABLE all the protections of "startup security" (to the extent that they can be "disabled")...
 
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ultrafiel80

macrumors newbie
Original poster
Feb 20, 2019
12
42
So here is a quick response...

  • The two machines I am using are a 2017 MacBook Pro (15-inch) and a 2020 iMac (27-inch).
  • My goal is to use the MBP as an external disk so that I can use the iMac essentially as the display (although the hardware is being used so it actually upgrades the MBP—except for the ssd speed through Thunderbolt 3)
  • I had to boot the iMac into macOS Recovery Mode (command-R upon startup), and then once logged in, go to the Utilities/Startup Security Utility menu. https://support.apple.com/en-us/HT208198
  • I then changed the settings for Secure Boot to "Medium Security" and the External Boot to "Allow booting from external media"
  • On the MBP I just set it to start up in Target Disk Mode.
  • When all those changes were done, I could plug the MBP to the iMac with a Thunderbolt 3 cable (make sure it isn't just USB-C), and select the disk of the MBP as the boot drive
  • After all of this I was able to boot from the MBP
There were some caveats/slight issues though:
  • When I finally booted from the MBP my iCloud account had to be re-authenticated
  • I had both drives show up as "Macintosh HD", so that made it a bit harder to pick the correct one to boot from (this only happens when I use the option key to select the boot drive upon restart)—this happens even though I had changed my drive name to something different like "iMac SSD"
  • I also have a bootcamp partition on the iMac for Windows, sometimes it would boot into that even though I didn't want that
  • I thought I hosed my iMac drive because I kept getting into Windows when I had detached the MBP; I'm not sure what happened there, but fortunately I did get back into my normal macOS and haven't had an issue since
  • I did have to authenticate access to each drive/computer because all of the storage is encrypted APFS; this wasn't too bad though
  • All-in-all I did have to restart numerous times before things got working, don't expect this to happen on the first time (unless I just wasn't very lucky)
Hopefully you can reproduce this, I haven't tried a second time booting to the MBP after the initial success, but I likely will today, or latest tomorrow. Good luck!
 
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VolceOntra

macrumors 6502
Apr 25, 2007
311
77
Unfortunately that doesn't seem to be working for me. Thank you for the follow up, however.
 
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VolceOntra

macrumors 6502
Apr 25, 2007
311
77
What hardware are you trying to use? I'm guessing an iMac for the display?

I'm using an iMac (2019) without the T2 chip as the display and a MacBook Pro (2019) with a T2 chip both on the latest Catalina.

I've also tried a 2017 MacBook Pro and the same 2019 MacBook Pro and that boots to the 2019 MacBook Pro disk refuses to accept my password when I enter it.
 
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