I believe that it would be lubricant from the hydrodynamic bearing[1] inside the hard drive.

[1: https://en.wikipedia.org/wiki/Fluid_bearing ]

Thanks, that was interesting to read. I take it that helium-filled hard drives work in a similar way?

Exact same way. The only difference is that helium is much less dense than air, so there's a lot less drag and thus the drive is more efficient.

That's just glue to hold the shielding plate on. You'll notice its nowhere near the bearings; in fact, it's on the outside of the case.

Phlogiston.