Time Machine hangs during "Preparing Backup"

I've trashed the .InProgress file, disconnected the external drive, reconnected it, run Disk Utility First Aid on it and come up clean. Disconnected the external again, shut down the laptop (2015 MacBook Pro running Catalina 10.15.6), restarted, reconnected, still getting hung up at Preparing Backup.

Addendum: switching USB ports might have made a difference. Anyway: resolved.