Time Machine couldn’t complete the backup on several occasions
I'm running Big Sur on a 2015 iMac retina. I have two external hard drives because eventually one will not backup anymore. Each time it fails to backup I need to erase it and start over, which takes several hours for the first backup. Eventually the other one will fail as well which then requires starting over. Sometimes they both fail at the same time which leaves me in a bind. These failures are unpredictable and sometime happens within a month of initial backup. How can I fix this?
iMac 27″, macOS 11.1