Unformatted disk space includes the gap, preamble and ECC space. The complete rotation of sectors on a platter at a given radial is called a track . Current disks have 50,000 to 100,000 bits per cm linearly on every track. The amount of data you can store on a given track is based on surface and air quality. Winchester disks are sealed at the factory to prevent dust.
Most disks have multiple platters stacked vertically with a single head for each platter.
A set of tracks at a given radial is called a cylinder . Originally sectors were arranged on disks with a certain fixed number of sectors per track at any given radial. Now the cylinders are divided into zones to allow for more sectors per track as the radial increases. Remapping bad sectors is also a function of the disk controller. A way to distribute software with the advent of the personal computer.