Error source drive cannot be found, but source drive is my hard disk

My Passport Ultra does not seem to recognize new files on hard drive.  It gives the above error, but then tells me that the backup was performed successfully.  When I look on the My Passport drive for new files that should have been backed up, they are not there.  How can I fix the problem?

Hello THeartPA, welcome to the WD Community. Try running a DLG test to verify there is no hardware damage. If the test completes successfully the problem would appear to be with the software used to create the backup. 

How to test a drive for problems using Data Lifeguard Diagnostics for Windows