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Fix_Preboot_High_Sierra.md

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Fixing Preboot partitions of APFS containers modified by macOS High Sierra

Symptom

On a multi-boot macOS system, High Sierra can interfere with booting newer macOS versions by modifying Preboot partitions. Specifically, it changes the ProductVersion in the SystemVersion.plist file on the Preboot volume.

In my case with a Comet Lake workstation, after using High Sierra and attempting to boot Big Sur, I encountered the "forbidden" symbol. Adding -no_compat_check to boot-args resolved the issue temporarily.

Initial solutions included:

  • Using -no_compat_check boot argument
  • Reinstalling the newer macOS version
  • Manually fixing via Terminal

This compatibility issue was uncommon and took until late 2024 to be fully resolved, despite early reports in hackintosh forums.

Fix

  • Download MacSchrauber's ROM Dump tool from his Github repo, specifically the Download.the.Dumper.from.github.zip
  • Extract the .zip
  • Run the "Download the Dumper from github" applet
  • Select "Download"
  • This will download Macschrauber's Rom Dump.dmg file (the applet also servers as a key to unlock this password protected diskimage so it can be mounted)
  • Once the .dmg is downloaded and mounted, navigate to /Volumes/Macschrauber's CMP Rom Dump/Readme & other tools/Preboot fixer and renamer to find the "PrebootFixer and Renamer"
  • Follow the MacSchrauber's Instructions

Important

Don't delete the "Download the Dumper from github" applet afterwards. Because otherwise you cannot mount the .dmg containing the ROM dump tools, since it's password protected

Best Practice

  • Simply don't install macOS High Sierra alongside newer versions of macOS!