Time Machine stuck on "Preparing backup" after 27.1 — here is the fix that worked [Test copy 40-34780]

Backup sat at Preparing for nine hours on a clean install. Disk Utility reported no errors and Console showed nothing useful.

What fixed it: unmount the drive, delete the .inProgress bundle at the root of the backup volume, then remount. The indexer rebuilds and the backup completes normally.

Posting because every thread I found said "just reformat", which is terrible advice.