FWIW utility generators are cooled with hydrogen to reduce windage losses and promote cooling. Hydrogen is probably lower viscosity and higher thermal conductivity than helium. It's also very hard to contain due to the small molecular size and has the widest flammability range of any gas with which I'm familiar, making it an interesting gas to work with. ;)

Isn't there also a potential problem with hydrogen embrittlement (depending on what materials are exposed) that you avoid with helium?

I don't know, but it seems possible. The only references I recall about hydrogen embrittlement were with steam boilers at high temperature and pressure. I suppose like most engineering solutions it comes down to the various tradeoffs between cost and effectiveness for hydrogen vs. helium.

The utility generators probably don't worry too much about leaks, since they can just refill with new, abundant hydrogen, too.

It would be amusing to have a hydrogen-cooled hard drive for which you'd have to regularly refill the water tank so that it could generate more hydrogen for itself. :)

Yeah, and it would have a little Bunsen burner to safely get rid of waste hydrogen!

Yes, it's the hydrogen that leaks out that's the problem. I was told that the way to find hydrogen leaks in plant piping was to walk the line holding a corn broom over the line. When it bursts into flame you found the leak. In an enclosed space it can form an explosive mix with air if it doesn't ignite before it accumulates.

He has a conductivity coefficent K (W/mK) = .151 and air = .026. Since everything else in heat flux eq is linear, approximately 5x rate of heat movement per degree that the HD is heated beyond surrounding air is expected.

Yeah, I can read a table of figures too, but what does that mean in the context of a hard drive? What percentage of the heat generated by the write heads (or whatever) is conducted away via air?

That is a function of the design implemented. Since you can read a table of figures you can probably also plug them in the heat flux equation in with your own guesses at air gap distances and surface area. The relevant info is that 5 times the energy per degree difference are conducted, that is why any hot spots from friction are smoothed.