Dutifully searched the forum and couldn't find an answer. Have a friend who is running a 2012 iMac with a WD external drive used for TM backups.
An ancillary problem occurred when his carrier, Verizon, decided to get out of the mail server business. No problem, but my friend got a little click crazy (I cut him plenty of slack, he's 76 years old and still learning!). Next thing you know, he couldn't connect to his mail server at all.

A final solution to his mess (not the real problem) was to use Time Machine to do a system restore about three TM backups back. The restore worked beautifully and he was soon back up and running.

All is fine with his mail, his iMac, his overall health. However, TM is having trouble now. Set to full backups, the backups fail now. I checked the external drive and Disk Utility gives it a green light.

But the backups aren't working. Any idea what I can do to get TM happy again?
Wolf Wallis