Platter Diameter
If you compare hard drives by only looking at their spindle speeds, you might believe that different products should perform similarly, but this definitely is not true. High spindle speeds are imperative if you want to get high performance, but the effective speed at which sectors pass the read/write heads varies considerably.
All hard drives rotate at constant angular velocity, which means that they are designed to stay at one rotation speed instead of adjusting the speed to the read/write heads as it is common in optical drives. As a consequence, the distance covered by bits stored in the outer platter areas in a second is much higher than on inner tracks. On the outside of a 3.5" platter, the track length is approximately ten inches, as opposed to 2.5" close to the spindle motor. At 7,200 RPM this results in an absolute velocity of ~67 MPH on the outside versus ~17 MPH on the inside of a platter. It is obvious why data transfer rates on the outside of a rotating disk are far higher than on the inside.
It is for this reason that defragmentation tools, which realign files that are scattered and fragmented across a hard drive, always place the Windows swap file at the very beginning of the storage medium, which is where the swap file performs best. Another conclusion of the absolute speed comparison is that a 2.5" hard drive can never reach the high data transfer rates of a 3.5" hard drive, because the effective data rotation speed isn't fast enough.
Platter Count
If you've already spent time dissecting a hard drive family and the properties of each drive, you'll know that the available capacities do not always correspond to a manufacturer's statement on per-platter capacity. For example, Seagate's Barracuda 7200.10 stores almost 200 GB per platter, yet there are versions with 250 GB and 320 GB.
The explanation for this can be found in market requirements. Some customers might specifically ask for a 250 GB hard drive, even if it is only for the sake of comparing different quotes. Cost pressure is another reason to offer certain capacity points: the average user might only be able to afford a 250 GB drive, and she or he might not even need more storage capacity either. It is obvious that hard drive manufacturers have to adjust to the so-called "sweet spots", and these have been at certain capacities. In the end, manufacturing yields are an issue as well, and selling a large number of drives is more important than fully utilizing the maximum storage capacity per platter for every product.
For these reasons, many hard drives do not utilize their full storage capability, which means that the slower inner tracks of products with an odd ratio of capacity and platter count (per-platter capacity) aren't used. While this crops the capacity, it also ensures better minimum transfer rates.
