Target Disk Mode Confusion

[VolceOntra]

I've been using TDM on other machines for a while, however I got two machines:

• MacBook Pro (2019)
  • FileVault off
• iMac 27 (2019 --- this is a brand new machine)

And I've been trying to put the MBP in target disk mode and boot the iMac using that disk.

However, some weird stuff is going on:

A) The iMac doesn't recognize the MBP all the time when I plug it in
B) When it does recognize it, the Startup Disk shows the name of the drive and says it is encrypted for a few seconds and then it gets removed and added back without the encrypted text (but FileValut is off -- is this the T2 chip?)
C) When I put the MBP in TDM and reboot the iMac holding the option key, it never shows up.
D) If I select the MBP Disk in the Start Up disk util and restart, it reboots, but eventually boots into the iMac disk.

I am using a thunderbolt 3 cable and have been able to use TDM between the iMac and other machines that I own. I'm at loss at what's going on, anyone know?

 

[tyc0746]

It sounds like need to disable secure boot on the MacBook Pro - as that wills top you booting from external drives.

 

[VolceOntra]

It sounds like need to disable secure boot on the MacBook Pro - as that wills top you booting from external drives.

Thank you for the suggestion, but I have disabled it.

What's also interesting is that a 2017 MBP can see the HDD of the 2019 MBP and boot into it, but it refuses to accept the password in the login window

 

[VolceOntra]

In case this is helpful to anyone else, Apple support is claiming this doesn’t work because of “driver” differences. Seems fishy. Equally so, they also claim it’ll work with Big Sur.

 

[danflynn]

I would suspect this has to do with the Secure token (https://derflounder.wordpress.com/2018/01/20/secure-token-and-filevault-on-apple-file-system/) A T2 Mac drive is ALWAYS encrypted regardless if FileVault2 is enabled. I just had this problem on a 2018 that wasn't encrypted and a 2nd admin account password wasn't unlocking the drive in TDM. I do know that it won't mount on non-TB3 devices. You could try and encrypt it and see if it will then mount when in TDM.

 

[VolceOntra]

I would suspect this has to do with the Secure token (https://derflounder.wordpress.com/2018/01/20/secure-token-and-filevault-on-apple-file-system/) A T2 Mac drive is ALWAYS encrypted regardless if FileVault2 is enabled. I just had this problem on a 2018 that wasn't encrypted and a 2nd admin account password wasn't unlocking the drive in TDM. I do know that it won't mount on non-TB3 devices. You could try and encrypt it and see if it will then mount when in TDM.

Thanks for the reply and suggestion.

Unfortunately, any combination of FileVault 2 being on doesn't seem to work either.

I am able to mount the disk when the iMac is booted.

I just can't boot into it and it doesn't show up as a boot disk when I restart the iMac and hold option. If I attempt to do disk from System Preferences, the MBP does show up as a startup disk, but the iMac will just reboot using its own disk.

 

[danflynn]

Oh then you need to change the "Startup Security Utility" on the iMac as it like every other T2 Mac does not boot to external devices by default.

Follow this need to boot into recovery to change and enter your password

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

Use Startup Security Utility to make sure that your Mac always starts up from your designated startup disk, and always from a legitimate, trusted operating system.

support.apple.com

What is the Startup Security Utility on Mac?

Use the Startup Security Utility to ensure that a Mac with an Apple T2 chip starts up from designated startup disk and trusted operating system.

support.apple.com

 

[VolceOntra]

Oh then you need to change the "Startup Security Utility" on the iMac as it like every other T2 Mac does not boot to external devices by default.

Follow this need to boot into recovery to change and enter your password

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

Use Startup Security Utility to make sure that your Mac always starts up from your designated startup disk, and always from a legitimate, trusted operating system.

support.apple.com

What is the Startup Security Utility on Mac?

Use the Startup Security Utility to ensure that a Mac with an Apple T2 chip starts up from designated startup disk and trusted operating system.

support.apple.com

The 2019 iMac doesn’t have a T2 chip,
only my MacBook Pro has it. The MBP does have the security turned off as well.