We already mentioned data density, but we should also talk about storage density, which is expressed in gigabits per square inch. This cannot be directly compared to the data density in gigabytes per platter, because manufactures do not always utilize the entire platter to store data. Also, the capacity per platter usually refers to a 3.5" hard drive, while the storage density in gigabits per square inch can be compared across different form factors. Storage density highly depends on the recording technology used.

Perpendicular Magnetic Recording (PMR) is the state-of-the-art recording technology today. Unlike conventional, longitudinal recording along the track, in this technique the magnetic elements are aligned vertically. This helps to reduce the risk of magnetic elements influencing each other, known as superparamagnetism, and it allows the storage of more bits on the same area to increase areal density. The hard drive industry is already hoping for a tenfold capacity increase in the long run, thanks to perpendicular recording. The first terabyte hard drives using PMR will be available soon, offering record storage capacities with high data integrity.

The future will bring Heat-Assisted Magnetic Recording (HAMR). In this technology, a laser heats up the surface in order to reduce the intensity of the magnetic field required to influence magnetic particles on the platters. This will allow further increases in data density, as the heat-assisted technology allows more precise manipulation of magnetized elements.

High data density is desirable, as it has a positive impact on data transfer performance: the more bits the drive can read concurrently, the faster it is. As a result, a new 3.5" 7,200 RPM hard drive always outperforms an older model. However, access time doesn't benefit from higher storage densities, as the head positioning cannot possibly be accelerated without putting substantial mechanical strain on the components.

Spindle Speed

A drive's spindle speed in revolutions per minute (RPM) is by far the most important parameter in assessing overall performance. A high spindle speed results in higher platter velocity, which means more data passing the read/write heads. The faster a drive spins, the more data it can deliver or store in a given time frame. But high spindle speeds also have a beneficial impact on access time: as soon as the heads are aligned over a track it usually has to wait until the required sectors pass underneath. Higher spindle speeds reduce this latency, although modern hard drives typically start caching data proactively while waiting for the right sector(s) to pass the heads. Even then the drive might still have to wait for a servo track, which is used to mark the beginning/end of a data track.

3.5" hard drives for servers and workstations spin at 10,000 or 15,000 RPM, while desktop drives normally work at 7,200 RPM. Only Western Digital's Raptor brings 10,000 RPM into desktop PCs; it can still be considered the perfect hard drive for enthusiasts. Still, it is very expensive from a cost per gigabyte standpoint, because you'll have to fork out more money for the 150 GB Raptor than for a 500 GB 7,200 RPM drive.

Notebook drives typically rotate at slower speeds: 4,200 RPM drives are being replaced by 5,400 RPM models even in some budget notebooks, but there are still only few 7,200 RPM notebook drives available. One reason for this is the energy requirement, which increases at higher spindle speeds. Portable computers usually depend on long battery run times, which is why system builders might hesitate to deploy 7,200 RPM drives into mainstream notebooks. 1.8" and smaller hard drives run at 4,200 RPM, while 1" and 0.8" models operate at even slower speeds.

New 3.5" hard drives at 7,200 RPM provide up to 90 MB/s of transfer speed off the medium, while 2.5" drives are considerably slower - in the area of 30-50 MB/s - and 1.8" models and smaller drives are much slower still.