B-grade could be anything from just an opened/damaged package (most HDD's are oem anyway... just an anti static bag, covered in bubble wrap) so i doubt its that. Could be a refurb, or a return for someone that bought the wrong one, not supported by their system, or other things...
If its a refurb, then it'll have been tested, so you shouldn't need to worry. B-grade stuff gives a good price difference sometimes. Totally worth it if its not critical!