Time machine backup failed
I'm on 10.11.6 and my TM backup has stopped working.
Can't even run it manually
Message says "Backup failed" and suggests using Disk Utility to repair; I run the utility, apparently successfully, but the error persists
Backup disk is WD My Passport external drive with plenty of free space
Wanting to backup before updating from El Capitan to Catalina
MacBook Pro 15", OS X 10.11