The vast majority of hard drives are fixed hard drives that are permanently sealed and fixed in the hard drive. Early hard disk storage media were replaceable, but today's typical hard drives are fixed storage media that is encapsulated in the hard drive (except for a filter hole to equalize the air pressure). With the development, removable hard drives have also appeared, and they are becoming more and more popular and varied. Most of the hard disks installed on microcomputers are called "Winchester hard drives", or simply "warm disks" because they use Winchester technology.