Hard drives are not sealed air tight. I know - I have taken enough dead ones apart to be sure of it.
There is always a breather hole somewhere with a filter of some kind.
Also, I remember reading somewhere that the air inside is required as it acts as a cushon for the read/write arms. At high RPMs the spinning air helps keep the heads from impacting on the disk surface.
My $0.02 on the subject...
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