When the problem first occurred I reformatted my backup disk to make it case-sensitive like the disk that I was backing up. Each time I change it, after a while it gets changed back to what it was before. For me the issue is how can case-sensitivity get changed all by itself after working properly for months. Mind you, during formatting I see nothing about case. The option that I select is "MAC OS Extended (Journaled)"
The specific error message that I keep getting is:
"A Time Machine couldnât complete the backup to âWD20EARSâ.
"A disk you are backing up is case-sensitive, but the backup disk is not."
"Open Time Machine preferences to select a different backup disk or to choose not to back up the case-sensitive disk."