Except for the helium-filled ones, they aren't sealed; there is a very fine filter that equalises atmospheric pressure. (This is also why they have a maximum operating altitude --- the head needs a certain amount of atmospheric pressure to float.)
Yup, this is why the label will say something along the lines of "DO NOT COVER DRIVE HOLES".
How does the helium stay in if it is not sealed? I am not familiar with hard drive construction, but helium is notoriously good at escaping.
I think he meant in general drives aren't sealed, except the helium ones are sealed.
Oh, I see. Makes sense. I wonder if dust really can infiltrate a drive? Hmm.
I"ve heard 2nd-hand that helium will slowly escape (over a period of years).
Don't know the details, but dust could have been impeding the effectiveness of his fans or clumping to create other hotspots in the system (including in the PSU).