Some pressure is required to keep the head flying above the surface of the drive. Removing all gas from the drive gives you a very nasty z-axis control problem.

I wonder if hard drives were part of Elon Musk's inspiration for the Hyperloop skids.

Same reason the US flag was 'flapping' in the 'wind' on the Moon. ;)

Without any air, there's nothing to stop the head from travelling without running into control issues. The gas molecules actually act as a buffer preventing too much movement. The head will flap about with the slightest motion like a flag in a vacuum.

Air pressure is 1kg/cm^2 ... so you will need to make a hdd that can operate under 400 kg of external load. Getting to the point of vacuum is expensive too. And hdd already require enormously precise manufacturing process.

You would need much better seals to resist the pressure difference.