I wonder if filling it with hydrogen an even lighter material would work better? I know Hydrogen is obviously more reactive then helium, I can' think it would significantly impact performance.

If a container can be sealed to hold helium, then a seal for hydrogen can't be too far off - and rather than just using it for the HDD's, methinks it would be a step forward in making hydrogen powered vehicles more readily available.

We don't have enough Helium to start filling hard drives with it also, use Nitrogen (like many motorcycle tyres), we have plenty, so we can leave the Helium for medical uses like NMR machines.

Hydrogen is normally a dipole molecule in our atmosphere (they pair together). So it's about 17% lighter than helium. Not much difference for something that would require a lot more safety in mass production. Although, on the same note, helium is also a finite resource, in much smaller quantity than hydrogen.

why not use vacuum??

Two of the inconvenient things about hydrogen can leak through solid metal; which damages the strength of the metals. Which is something NASA and others finds immensely annoying. So you can put hydrogen into an HDD, but it won't stay there. So if your drive requires hydrogen to be there (and stay out of the electronics) to operate reliably, then it probably won't operate reliably very long.

Helium is expensive, in short supply, and getting more expensive by the day. Can't they just pull a vacuum instead?

Pulling a vacuum is what you do when you don't want to get rid of heat. It would create a thermal management problem.
Helium is mined with natural gas. It can also be manufactured in a Thorium Molten Salt Reactor.
If the price of Helium rises (pun intended), then it will be used only for important things, as opposed to party ballons.

Helium is depleting/ we are running out of Helium as a resource. Consequently, using Helium for this proposal is a bad idea. Instead, Wester Digital should focus their efforts on enhancing SSD-technology.

Methane would be a good substitute. It is extremely plentiful and lighter than air. The only worry would be an "ignition" lighting the gas. A fail safe could be designed to release the gas if it did ignite somehow.