Hello,
It is possible the partition is corrupted and that’s why it says it is full when actually it is not.
You can try formatting the drive. This will delete the partition and create a new one. Note that this will also delete all the data stored in the drive so if you have anything important go ahead and save it to another location.
How to partition and format a WD drive on Windows (8, 7, Vista, XP) and Mac OSX
