Hi again, QuestinMarks!
If the diagnostic tool says there is a problem, you should probably contact the HDD manufacturer's technical support and ask for assistance. If your external is still covered by the warranty, you should be able to RMA it and get a replacement from them.
As for changing the drive letter, it's a safe way to try to get your EXTERNAL HDD to be recognized and detected by Windows. (Though, keep in mind that this procedure works like that only on externally connected drives!).
Since you can still use your drive, I'd recommend you to back up all your data from there and consider getting in touch with your HDD's support.
Good luck!
Cheers,
SuperSoph_WD