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Jun 9, 2021

Downgrade MacOS without second mac (M1)

Heads up: This is from the macOS Monterey beta era (2021). The swcdn.apple.com InstallAssistant link below is version-specific and has almost certainly expired — before running the curl step, grab a current InstallAssistant.pkg link for the macOS version you want from an up-to-date source (e.g. Mr. Macintosh's full installer list). The core technique — swapping the SharedSupport payload — still holds.

After flashing macOS 12 (Developer Beta), I ran into issues with Xcode and Homebrew. I needed to go back ASAP, because I needed Xcode for a school project.

I was dumb enough not to download Big Sur from the App Store while I was still on macOS 12 — I went straight to recovery mode and erased my disk. Then I discovered I couldn't choose which macOS version to install; the only option was to reinstall the developer beta. After reinstalling it I couldn't create an account, so I'd soft-locked myself. And without a second Mac, I couldn't make an installation USB. Here's how I got out of it.

Downgrade from macOS 12 beta to Big Sur

  1. Turn off your Mac, then hold the power button until Options appears. Click it.
  2. Erase the main disk using Disk Utility.
  3. Connect a removable drive (an external SSD or a USB flash drive).
  4. Open Terminal and run:
cd '/Volumes/<name of your removable drive>'
mkdir -p private/tmp
cp -R '/Install macOS 12 Beta.app' private/tmp
cd 'private/tmp/Install macOS 12 Beta.app'
mkdir Contents/SharedSupport
# Replace this URL with a current InstallAssistant.pkg for your target macOS version
curl -L -o Contents/SharedSupport/SharedSupport.dmg http://swcdn.apple.com/content/downloads/55/59/071-00696-A_4T69TQR1VO/9psvjmwyjlucyg708cqjeaiylrvb0xph94/InstallAssistant.pkg
./Contents/MacOS/InstallAssistant_springboard
  1. Something strange happens: it looks like it's reinstalling the beta, but it actually installs Big Sur.
  2. After it installs, shut down and return to the recovery options.
  3. Open Terminal and run:
resetpassword
  1. In the dialog that appears, choose Erase Mac from the menu bar and confirm (the blue link).
  2. The Mac reboots back into recovery mode. Install the "fake" Big Sur again.
  3. After installing, you can finally log in and create a recovery USB. (You'll need to reinstall the system once more to clear "Reinstall macOS 12" from the recovery menu.)

Thanks to