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Track Density and Areal Density
The track density of a hard disk refers, unsurprisingly, to how tightly packed the tracks are on the surface of each platter. Every platter has the same track density. The greater the track density of a disk, the more information that can be placed on the hard disk. Track density is one component of areal density, which refers to the number of bits that can be packed into each unit of area on the surface of the disk. More is better--both in terms of capacity and performance. See this page for a full discussion of density issues.
The earliest PC hard disks had only a few hundred tracks on them, and used larger 5.25" form factor platters, resulting in a track density of only a few hundred tracks per inch. Modern hard disks have tens of thousands of tracks and can have a density of 30,000 tracks per inch or more.
The chief obstacle to increasing track density is making sure that the tracks don't get close enough together that reading one track causes the heads to pick up data from adjacent tracks. To avoid this problem, magnetic fields are made weaker to prevent interference, which leads to other design impacts, such as the requirement for better read/write head technologies and/or the use of PRML methods to improve signal detection and processing.
Next: Zoned Bit Recording