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?