DOWNGRADE MOJAVE TO HIGH SIERRA PRO
Right now I'm considering this a "test" machine, and will be running it through the workflows I need to ensure there is no latent instabilities caused by the downgrade.įor this experiment, I decided to leave Mojave installed on the primary Container Volume, and used Disk Utility to add a second APFS Container called "HighSierraTest".īefore doing anything else, I booted the iMac Pro into Recovery, disabled the T2 security, and enabled boot from external drives. I was able to successfully downgrade one of the iMac Pros to High Sierra 10.13.6. All the information I posted is based on past knowledge and helpful discussions like the Apple discussion thread posted above. For the record, while I have downgraded machines in the past to something earlier than what it shipped with, I haven't had a need to do it in quite some time and I haven't done it to any of the newer Macs that have T2 chips, but the T2 chip in itself shouldn't prevent anything once you disable the necessary features. Then you should be able to build a boot disk off of it and proceed.ĭidn't always used to be this complicated but I suppose this is the new norm. You may need to find a user who owns an iMac Pro that originally shipped with High Sierra as they should be able to download the correct installer.
So with that said, the trickiest part of your journey may be getting a machine specific build of High Sierra for an iMac Pro. If you look at the thread above a user was only able to pull this off on their 2018 MacBook Pro by pulling down the High Sierra installer on a 2018 MacBook Pro that originally had High Sierra, so it pulled down the machine specific build that would allow him to boot HS on a MBP that shipped with Mojave.
DOWNGRADE MOJAVE TO HIGH SIERRA INSTALL
It used to be you could just get a regular installer of that OS and install it, but according to the thread that u/DigDugteam posted it appears Apple is pushing machine specific builds when you retrieve installers from the App Store that can complicate things. Since the iMac Pro originally came with High Sierra, theoretically you can downgrade it. While Apple doesn't advertise it, the general rule of thumb is you can downgrade a computer to whatever OS it shipped with when it was first released, not necessarily what it came with when you bought it.
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