There is a physical reason why I/O speed doesn't scale with density. When the density of data on disks gets denser, it does so on both axes - x and y, or radia

There is a physical reason why I/O speed doesn't scale with density. When the density of data on disks gets denser, it does so on both axes - x and y, or radial and tangential. But read/write heads only access one track at a time, so the speed-up is only along one axis. Think about CDs, DVDs, and BDs. The discs can only be spun at a finite rate (somewhere around 10k RPM) before they shatter. The R/W head can read the data within any single track at full speed, but only one track at a time. From CDs to BDs, the amount of data per track increased (this doesn't affect the amount of time needed to read all the data from a disc), but also the number of tracks increased (this does affect the total time).