Zone Bit Recording (ZBR) is used by disk drives to store more sectors per track on outer tracks than on inner tracks. It is also called Zone Constant Angular Velocity (Zone CAV or Z-CAV or ZCAV).
On a disk consisting of concentric tracks, the physical track length (circumference) is increased as it gets farther from the center hub. The inner tracks are packed as densely as possible, but with a CAV drive the data on the outer tracks are less densely packed. Using ZBR the drive divides all the tracks into zones and the inner track of each zone is packed as densely as possible with the other tracks in that same zone recorded with the same read/write rate. This permits the drive to have more bits stored in each track than not using this technique. More bits per track equates to more sectors per track.[1]
On a hard disk using ZBR, the data on the tracks in the outer most zone will have the highest data transfer rate. This is where the operating system typically stores its own files. Testing disk drives when they are new or empty after defragmenting them with some benchmarking applications will often show their highest performance. After some time when more data is stored in the inner tracks, the data transfer rate in those zones will be slower; often making people think their disk drive is slowing down over time.[1]
Some other ZBR drives, such as the 3.5" floppy drives in the Apple IIGS and older Macintosh computers, spin the medium faster when reading or writing inner tracks.