Air is required for the heads to function. While the disk's internal environment is separate from the outside air to keep it clean, air exchange is permitted between the outside and inside of the drive to allow the drive to adjust to changes in air pressure caused by thermal expansion. A special "breather" filter is installed to prevent foreign matter from contaminating the drive.

The arm of the drive is a tuned mass damper attached to a voice coil that utilizes ground effect to maintain the a precise distance between the head and the platter. Spinning disks wouldn't work in a vacuum using current technology, because there'd be no ground effect to lift the arm off the platter.

I did some everesting and our organiser had tired regular laptops and they always packed up through disk crashes. I guess that would be at about 16 to 20,000 feet, or about half sea level pressure. That was in pre SSD days.