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How to fix Big Sur corruption while dual booting an older macOS version.

For those who dual boot multiple macOS versions this might be helpful. I specifically had this issue with dual booting High Sierra and Big Sur. When booting High Sierra, it attempted to mount Big Sur and corrupted the volume and made it unbootable. (It took me like 3 weeks to see this correlation lol.) To fix this issue you’ll need to follow these steps. This little guide will basically tell your High Sierra partition to not mount the Big Sur volume at boot allowing you to boot to High Sierra without damaging Big Sur.

While in High Sierra or your older macOS:

1.Create a new APFS partition for your Big Sur installation. (You can try keeping them in the same APFS container, I haven’t tried but I would recommend having a different partition as that should keep it cleaner and less likely to corrupt each other)

  1. Once that is made right click on the partition you just made in disk Utility and copy the file system UUID. You’ll need this for the next part.

  2. Open terminal and type ‘sudo vifs’. Type in your password when prompted. This will open your system fstab file. Which is a small file that tell your OS what to do while booting.

  3. Use the arrow keys to navigate to the bottom of the file and click ‘i’ to edit. Type the following

“UUID=‘the uuid you copied here’ none apfs rw,noauto”

For example mine was:

UUID=EA8072E1-3553-43D7-B4D7-B5F9EB9BD3C6 none apfs rw,noauto

  1. Once that is typed out, click escape. Then, type “wq!” to exit the editor.

  2. Once that is finished, attempt to reboot to the older macOS. If the partition you made for Big Sur is not mounted you succeeded!

  3. Continue your installation of Big Sur to the partition you made in the older macOS. After that’s all set up there’s another ‘update’ volume created that the older macOS sees. To keep the installation clean do the same process grabbing the uuid and typing in the uuid into the fstab file.

Remember not to mount your Big Sur partition in disk utility on the older macOS. This will most likely corrupt your installation of Big Sur.

I hope this helps somebody as I had this issue for weeks.

submitted by /u/Holy_Hobo_
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