One way that capacity and speed have been improved on hard disks over time by improving the utilization of the larger, outer tracks of the disk. The first hard disks and their controllers couldn’t handle complicated arrangements that changed between tracks. As a result, every track had the same number of sectors. The standard for the first hard disks was 17 sectors per track.

The tracks are concentric circles, and  the ones on the outside of the platter are much larger than the ones on the inside, typically double the circumference or more. Since there is a constraint on how tight the inner circles can be packed with bits, they were packed as tight as was practically possible given the state of technology, and then the outer circles were set to use the same number of sectors by reducing their bit density. This means that the outer tracks were greatly underutilized, because in theory they could hold many more sectors given the same linear bit density limitation.

To eliminate this wasted space, modern hard disks employ a technique called zoned bit recording(ZBR), also sometimes called multiple zone recording or even just zone recording.

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