There seems to be a lot of nonsense "floating" (pun intended) around here.
Most hard drives are NOT "sealed" at all. In fact, most have a tiny vent hole specifically designed to allow air in and out as the atmospheric pressure changes. This change in air pressure is very common - for example, for travelers who fly and change altitude. Our ears pop, hard drives vent through those tiny holes. They are filtered to prevent dust but make no mistake, they are NOT sealed, hermetically or otherwise.
Helium drives are sealed, however. This is to prevent the helium from escaping. Helium is good in this usage because being much less dense than air, changes in altitude do not result in any significant change in pressures, that may affect the drives performance.
As mentioned above, helium does not promote corrosion and being less dense, allows the platters to spin while encountering less resistance.
Once you pop the lid, the drive becomes useless.
Why?
If you break the seal and the helium escapes, that alone will not cause the drive to fail. What will cause the drive to fail, however, is the bolder size (microscopically speaking) dust particles that would get inside.
As for lower weight - I am amazed how this nonsense is being spewed, even by those you would expect to know better. A hard drive does not have 100 gallons of empty space to fill with gas!!! Yes, helium is lighter than air but just how much space do you suppose is inside a hard drive? A few CCs, maybe?
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