Plastic or Metal enclosures, if properly ventillated (ie. sufficient
airflow), won't matter.
BUT, if you buy an enclosure that doesn't have enough ventillation (eg.
AVOID ME-720 series 3.5" Plastic enclosures!!!!), you will kill the drive.
Now, given that WD made and sold the drive themselves in the external
plastic box, you'd think they'd test it already to make sure there's no
problems -- here, you'll just have to trust the drive maker that they're
big enough of a company already to do that. (In other words, because
it's a brand-name, I wouldn't worry much.)
But Metal enclosures are better overall -- why? the heat dissipates
through metal far faster than any plastic, so even passively, you'll
have a cooler interior (in general) with metal enclosures (assuming
exact same design, but sub'ing metal for plastic).
