My Book is a source but not a destination for backups

I have a 6 TB external My Book. Windows 10. I also have a My Cloud Mirror. The files from the My Book have been backed up to the My Cloud Mirror for months using Smartware.

Suddenly the My Book is shown only as a source for backups, but not a destination. I back up some things on my C: drive to both the My Book and My Mirror. I can no longer do this. I can backup from the My Book to the My Mirror, but I can no longer backup from C: to My Book.

This is a recent My Book WDBBGB / 25EE. The My Book shows up in Windows Explorer just fine, and I can manually backup files to it from C:, but Smartware used to do the backups automatically.

Thanks.