Time Machine stuck on "Preparing backup" after 27.1 — here is the fix that worked
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.
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Unmount the drive, delete the .inProgress bundle, then remount. Works every time on Sequoia.
That did it, thank you. Marking the thread solved.
Worth checking Spotlight indexing on the destination volume too. If mdworker is pinned it stalls the snapshot.