A hard drive is a dirt free environment inside an airtight case, which is critical for avoiding damage to the parts inside. A number of discs, called platters, spin around on a spindle. There can be from one to five platters in a single drive, although most drives just have one. For each platter there is a little arm with a read/write head on the end. There is one arm for each platter, and they reach out over the platter, much like a record player and the head changes the data on the surface. These heads never touch the platter, but float ever so slightly above them.



All drives have this basic design, but utilize other technologies to get the most from the drives. Notably there is often very little difference between competitors when using comparable systems. Drives with the same specifications, from two different manufacturers, will not very much in performance.

Drives come in a few different sizes. The standard desktop hard drive is one inch in height, while models for laptops are 9.5mm or 12.5mm, with the latter becoming too big for current laptop designs. There are also proprietary sizes available.

The number of platter depends on the use of the drive. Most drives only have one platter, which reduces the number of moving parts, namely the arms and heads, resulting in less chance of drive failure. Multiple platters allows for greater storage capacity. Laptop drives can have up to two platters. Desktop drives have 3.5 inch platter, those for enterprise computers or workstations look like 3.5 inch drives, but have 2.5 inch platters inside. Mobile drives have 2.5 and 1.8 inch platters, while some micro-drives use 1 or 0.8 inch platters.

Spindle speed, which is how fast the platters spin in the drive, makes a huge difference to the performance of the drive. Think about it, the faster it can spin, the faster the information that is being looked for will pass beneath the read/write heads, it's just quicker. Server and workstation drives often spin at 10,000 or 15,000 RPM, while desktops typically spin at 7200 RPM, although the WD Raptor is a notable exception that spins at 10,000 RPM. However the price of a Raptor is very high and you pay more for its 150 GB model than for a 500 GB drive.

Notebook drives spin at 4,200 RPM, which is slowly being replaced by 5,400 RPM models. Smaller mobile drives operate at 4,200 RPM, while the micro-drives operate even slower.

The cache stores frequently used data so that it can send data straight from there, rather than needing a read from the surface of the platters. This gives great speed improvements for frequently accessed data. Drives with a cache will give faster performance. Up to 16 MB is offered on some drives, but 8 MB has been found to be the most worthwhile, as 16 MB provides little extra performance over 8 MB.

The performance difference within a family of hard drives is very little. The biggest differences are found by moving up to 10,000 RPM, but with an obvious price penalty. For best value per GB, divide price by disc size and go from there.