Over the years, there have been many varieties of reel-to-reel tapes, tape cartridges, tape cassettes, hard disk cartridges, optical cartridges and floppy disks. Today, the industry has standardized on SATA hard drives, SATA optical drives and USB flash drives. See SATA and USB drive. The physical shape is half the standards issue, and most of the time, the media cannot be inserted into the wrong drive. The other half is the recording pattern, which is invisible to the human eye and is written onto the tape or disk by the drive. For example, a flash drive can be inserted into any USB slot on any device; however, if the drive was formatted for the Mac, it cannot be read by a Windows PC without a special program. See format program, magnetic disk, magnetic tape, optical disc and standards.