Incomplete backup

After backups keep getting error message: “Some files failed to backup”. Why am I getting this
and how do I find files that did not back up? Running Win 10 Pro with a My Book 3T.

How do you backup your files into your WD My Book? What is the name of the application used?

I use WD Smartware.

If so, this particular message relates to files that are in use or blocked by another process. You can confirm which files were skipped in the Backup Tab. There should be an exclamation icon which displays pending files.