Dust, fingerprints, etc.
Allegedly they may check these things under black lights at the factory for any evidence of opening to attempt to deny warranty. With drive prices having doubled since I bought it and all capacity bought out they'd be extra eager to find any excuse to attempt to deny. Magnuson–Moss warranty act should prevent them from doing that but I don't have a high power law firm on retainer to sue and intimidate western digital into compliance if they tell me to pound dirt.