Time Machine couldn't complete the backup This is the 2nd time I got this error message:



On my Mac Pro, I use two HDs for Time Machine backup. Time Machine would use them alternatively. The other HD works fine. With this HD, I used Disk Utility to perform "First Aid" and no problem was found. However, the Time Machine still cannot back up to this HD.

On the previous error, I had to reformat the HD. Any suggestions on what I should do this time?
It turned out that App Cleaner is the root cause of the problem. I used App Cleaner for quite some time and have purchased the premium version. My problem with App Cleaner started with its V6.0 version and I was still on High Sierra. So it was not a Mac OS problem.