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Because of the mobility of my work I decided to create a boot volume on an external USB-C SSD so that I can run my user environment on a number of Macs (on an iMac Pro (2017) while at work, on a 15" MacBook Pro (Retina, mid-2015) while on the road, and on a 27" iMac (2017) when at home). Unfortunately, while the 27" iMac and 15" MacBook Pro boot up fine from the external drive, the iMac Pro doesn’t.

I’ve tried resetting the NVRAM and even the SMC for good measure. The external boot volume contains macOS Mojave 10.14.1, and this should be fully compatible with the iMac Pro.

What am I missing here?

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From what you’ve said, I agree it should work fine.

My first guess would be to check the Startup Security Utility on the iMac Pro, a feature of the iMac Pro (and other T2 Security Chip enabled Macs) that allows Admin users the ability to enable/disable Secure Boot and also the option of allowing/disallowing External Boot. It’s the External Boot configuration I think is causing your problem.

To access the Startup Security Utility, follow these steps:

  1. Switch on the iMac Pro and as soon as the Apple logo appears, press and hold Command (⌘)+R
  2. The iMac Pro will boot into the macOS Utilities window
  3. Go to Utilities > Startup Security Utility from the menu bar
  4. When prompted, click Enter macOS Password to choose an administrator account and enter its password
  5. The Startup Security Window will appear and the bottom section provides two options in relation to External Boot. You will need to select the Allow booting from external media option
  6. Exit the Startup Security Utility
  7. Restart your iMac Pro and try booting up from your external boot drive

NOTE: The above steps require you to enter an Administrator password. If your workplace has not set you up with an Admin account you will need to ask them to follow the above steps.

Further reading:

  • Thank you for taking the time to answer. This sounds very promising! It's almost midnight here, but I will test this in the morning and will be sure to let you know. – user312934 Dec 5 '18 at 10:53
  • Yes, please do. I'm very confident this will solve your problem, especially since your disk successfully boots two other Macs. NOTE: I've updated my answer because I remembered the iMac Pro was a work computer. The links to further reading are just in case you need to show someone there what you need to do and to satisfy them with sources from Apple that it's legit. All the best with it! :) – Monomeeth Dec 5 '18 at 13:01

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