I had a problem with Migration Assistant and searched for a solution on the internet and found nothing. I suspected a solution, but the Apple Geniuses could not confirm if I was correct. Since I now have a solution, I thought I would document it for the next person.

I have a hardware issue on a 2020 Intel MacBook Air. It passed the Genius hardware test so they recommended a clean install of the software. I had a Time Machine backup, so I told them to wipe it and reinstall the operating system. They did not reinstall the operating system but rather upgraded me to Big Sur.

Here is the problem: Big Sur Migration Assistant, in the setup process, could not see my Catalina created Time Machine backup.

For completeness here are some other observations:

  • Big Sur Migration Assistant can see a Big Sur created Time Machine backup from a secondary computer.
  • Finder on another computer can see both backup directories.
  • Internet recovery on the MacBook Air could see all the Time Machine backups.
  • The OS version of the Catalina backups were “unknown.”

My hypothesis: Big Sur Migration Assistant is not backward compatible with Catalina-created Time Machine backups. And Apple did not document this change, nor inform their Geniuses.

The solution: A new clean install, but this time installing Catalina. Catalina’s version of Migration Assistant can see both my Catalina and the Big Sur created backups. Accept the hypothesis and start looking for a new backup routine that documents breaking changes.