Hydrogen?
If the reduced density of the gas surrounding the drive is the issue, why not hydrogen?
It is not an inert gas as Helium is but is is even lighter/less dense and in a hermetically sealed enclosure is not going to cause a fire if it is a pure hydrogen atmosphere. Helium is a bit pricier and I hope includes some process of gas recovery otherwise spinning disk prices will start climbing again to keep step with the capacity increases.