I have a WD Passport 2TB USB drive with a file on it I could not read, so I downloaded the WD Dashboard and ran the S.M.A.R.T. Diagnostic Extended Test and it found nothing wrong with the drive.
I ran Windows 10 chkdsk /r and it found a bad cluster in the file and replaced it. It found 21 other bad clusters in unused space and marked them.
It seems the WD test is unreliable.