Data transfer rates are somewhat less than for magnetic disks. Current CD drives read at around 3 to 6 megabytes per second, and current DVD drives read at 8 to 15 megabytes per second. Like magnetic disk drives, optical disks store more data in outside tracks and less data in inner tracks. A tape is kept in a spool, and is wound or rewound past a read–write head. Moving to the correct spot on a tape can take seconds or even minutes, rather than milliseconds; once positioned, however, tape drives can write data at densities and speeds approaching those of disk drives.