I have an iMac running High Sierra version 10.13.6. Attached via the USB port is a 4 TB HDD with a 1 TB partition I use for backups. I've been using Time Machine with the TimeMachineEditor app for almost a year now and its been working fine. Using TimeMachineEditor I have a backup scheduled for once a week. A day or so ago the scheduled backup failed and I can't seem to get it to work.
My earlier backups have been running around 140 MB and I have almost 300 MB on my backup drive. When I go to Time Machine Preferences I get a window like the one shown below.
The blue circle is used to indicate the red message circle. When I click on the red circle I get the following message:
I've run Disk Utility/First Aid on my TimeMachine backup drive and it appears to be ok. Below is an image of the First aid result.
When Disk Utility sees this drive it reports the following:
Can anyone explain what happened and why time machine will not finish the backup.