I use Mac OSX’s built-in Time Machine for backups, and a couple of times I’ve noticed my backups failed and couldn’t be completed successfully. I was unable to fix the problem until I reformatted the backup drive. Today I think I stumbled on the solution.
The problem seems to come from swapping a directory and a symlink to it. I had a set of directories with symlinks back and forth, and sometimes I moved the directory to where the symlink was and made a symlink to it from where it used to be. That seems to confuse Time Machine mightily.
Just removing both the symlink and the directory seems to “fix” it. Well… maybe it’s not a fix, but it’s better than the nuclear option of reformatting the backup drive.
Hope this helps someone.