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 be

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.