[dnanian]

It's an easy mistake to make. If your external drive fails in weird ways, the OS never tells the application, which will continue to copy to the old "Mount point".

A "mount point" is a special kind of folder that represents an external drive. But, if it converts into a "real" folder, it's hard to tell -- and applications keep copying, as Backup did.

It's safe to delete the files, though you might have to reset Apple Backup, if you want to keep using it.

Hope that helps!