I really need to run macOS High Sierra 10.13 as my primary OS natively (not via virtualisation etc).
Is it possible to downgrade a 2018 MacBookPro using a USB installer to macOS High Sierra 10.13 ?
Would it have the required drivers etc?
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2018 MacBook Pro originally shipped with macOS High Sierra 10.13.6.
It is possible to downgrade it to run the said release of macOS High Sierra on your MacBook Pro.
You can install macOS High Sierra 10.13.6 by downloading it from the Mac App Store, creating, booting and installing it via a USB installer. Alternatively, you can also use Internet recovery to directly download and install it natively.
High Sierra can be downloaded from Mojave by selecting Get macOS High Sierra on the Apple website How to upgrade to macOS High Sierra. High Sierra appears in the App Store as shown in the image below.
The download will appear as a software update, as shown in the image below.
When finished downloading, you will get the popup message shown below.
The installer will appear in the Applications folder, as shown in image below.
The instructions for creating the USB installer can be found at the Apple website How to create a bootable installer for macOS. Basically, there are two steps:
Use the Disk Utility application to erase the flash drive. Choose the following options.
Enter the following command in a Terminal application window.
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Once step 2 completes, you can boot from the USB installer by restarting the Mac and immediately hold down the option key until the Startup Manager icons appear. The USB installer will appear as one of the choices to boot from.
Apple documents the things to check if you ever manage to install or bring an older OS to newer hardware where driver support isn’t complete or even available:
If you don’t have a record of the OS that shipped, you can make a backup, erase the hard drive and then use Internet Recovery to ask for the proper version that shipped with your serial number. Apple does ship newer hardware within the same marketing name from time to time, so contact Apple Support if you need to know the version of the OS that shipped with a specific serial number.
Hold Shift-Option-⌘-R to get the original version (or the closest newer version past what was original if the original is no longer available from Apple content delivery network that powers Internet Recovery.
This is not a downgrade, you erase everything and walk away from an easy migration of a newer backup to a clean installation of an older OS. Many files and apps work well by manually migrating, but the official tool will want to force you to upgrade to the version that made the backup if you ask it to migrate data to an older OS build or version.