Then that would imply there is indeed some errors on the drive, eventually it should sort itself out, because the drive is big this can take a long time even when only doing a /f.
Another software you will want to use is Western Digitals Data Lifeguard i use it myself on my 2 hdds among other things...
This software will know just how messed up the drives are for sure, just be careful not to press erase.
As a Western Digital user i imagine you would also be entitled to use the free special edition of Acronis true image that only works with WD drives as part of the deal, it won't fix your problem but it can clone WD drives and its free...