2018 MacBook Pro won't let me install macOS High Sierra 10.13 from USB installer I've got a 2018 MacBook Pro (Model ID : MacBook 15,1) that shipped with macOS Mojave 10.14, that I need to downgrade to run macOS High Sierra 10.13.

I've tried reformatting the main drive using Disk Utility and then re-installing the OS from recovery mode, which from past experience loads the lowest OS the machine shipped with (for the 2018 MBP that is 10.13.6). But for some reason when I do this it keeps loading macOS Mojave 10.14.

So I've gone for another solution: I've created a USB installer of macOS High Sierra 10.13, wiped the MacBook Pro drive again, gone to the Startup Security Utility, and set medium security and allowed boot from external drive. I then restart the computer holding the Option key and booted using the USB installer.

After a loading screen, I keep getting a Install macOS Mojave menu. Any ideas why this would be, and why it's not letting me install macOS High Sierra 10.13?

Although the 2018 MacBook Pro originally shipped with macOS High Sierra 10.13, could it be that I have a later model of 2018 MacBook Pro, that only has drivers for macOS Mojave 10.14 and later? Or do all 2018 MacBook Pros work with the same drivers?
Original answer

As described in this support article, there are different ways to load macOS Recovery: 


  
  cmd+R: Install the latest macOS that was installed on your Mac.
  alt+cmd+R: Upgrade to the latest macOS compatible with your Mac.
  shift+alt+cmd+R: Install the macOS that came with your Mac, or the closest version still available.
  


I suggest you try the last option and see if it lets you install macOS 10.13.



Update as per OP comments

Apparently the key combinations from my previous answer did not work for you and you receive the message "/Applications/Install macOS High Sierra.app does not appear to be a valid OS installer application.", when creating the USB installer.

It seems you face exactly the issue discussed here. The AppStore downloaded only a down-sized version of the installer image. Try the solutions offered in the answers over there. Basically you have to first download the full image / SharedSupport folder before you can create your USB boot installer.