You have 2 choices.
- Start a new backup, which means wait while the old backups are purged from the NAS, then wait while the 400GB of data is backed up over the LAN to my ReadyNAS
- “Back up later”. Ignore the problem and hope it fixes itself over the next day. And while you wait, you have no backup
Thank you apple, for the choices. Grumble grumble.