MacOS time machine diagnostics

Time Machine failing: “an error occurred while copying files” ¹

log show --predicate 'subsystem == "com.apple.TimeMachine"' --info --last 4h|grep -i error

# or
log stream --level debug --predicate 'subsystem == "com.apple.TimeMachine"'

Hold down the Option key on the iMac while you click on the Time Machine “clock” icon at the top of the screen. Click Verify Backups to see if the procedure will run.

If no luck, hold down the Shift key on the iMac while you click on the Time Machine “clock” icon. Click Back up with Consistency Scan to see if things will run that way. ²