How Hard Drives Work
A hard drive consists of magnetically coated rotating disks enclosed in a sealed drive unit.
Computer software (the instructions that run the computer) is made up of discrete bits of data that is either a 0 or a 1 (really).
These 0's and 1's are stored on your hard drive magnetically and read by tiny read/write heads that are positioned over each side of the rotating disks.
The platters are spun at speeds between 5400 and 15000 RPM (revolutions per minute). The read/write heads move across the outside surfaces of the platters as they rotate to access the data.
Rotational Speed
The faster the platters spin the faster data can be retrieved from storage. Typical best hard drives have speeds of 7200 or 10000 RPM with 7200 RPM being the most common. The recommended minimum acceptable speed is 7200 RPM.
Buffer/Cache
Another factor to consider is cache. Cache and buffer mean the same thing. Cache is RAM located onboard the hard drive to temporarily store data as it is retrieved from or written to the hard disk. Files that are accessed repeatedly are then read from this memory. That reduces data access time as the read/write heads are electro-mechanical and are slow compared to buffer memory. Cache is measured in 4-8 MB chunks. Most quality hard drives will have 8-16 MBs of RAM as a buffer. The more cache you have the better your hard drive's speed and performance wil be.
Storage Capacity
The best hard drives range in size from 40 GB all the way to 750 GB. Drives in the 250 GB range are in the "sweet spot" in terms of capacity and price. The unit of measure is gigabytes (GB) which is 1,000,000,000 or 1 billion of the 1's and 0's mentioned previously. Just like cache the more hard drive capacity you have the better. Hard drive capacity is like a closet. You will always wish it were bigger.
Choosing a Hard Drive Interface
There are three types of hard drive interfaces in common use today.The first and most common is the IDE interface. Integrated Drive Electronics or IDE for short is also known as parallel ATA (advanced technology attachment). IDE data throughput speed can reach 133 Mbps. The hard drive is attached with a 40 pin parallel ribbon cable to the motherboard. One drawback to the parallel ATA interface is that these cables are limited to 18 inches in length and may impede air flow in the computer case, which is not a good thing. This may be mitigated by using the round PATA cables.
SCSI or small computer systems interface is an even older technology than IDE. Once used largely by business servers, SCSI (pronounced skuzzy) is a fast parallel interface with drives spinning at 10-15 thousand RPMs. Pretty pricey and technically hard to set up, but fast and reliable data storage. Data throughput in the 160-320 Mbps range.
SATA is a relative newcomer to the hard drive interface scene, but is rapidly overtaking parallel ATA as the interface of choice. SATA offers data transfer rate speeds of 150 Mbps +. Cables can be up to 3 feet in length and do not impede air flow inside the computer case. A new generation of SATA has recently been introduced. SATA II offers speeds of 320 Mbps. SATA is definitely shaping up to be the next hard drive interface of choice for serious computing.
Conclusion
It's recommended that you buy as large a PATA or preferably a SATA drive as your budget will allow.
SCSI is not recommended. In a nutshell, the best hard drives have a large capacity, 7200 RPM platter speed, minimum 8 MB buffer, and interface which will work with the motherboard you choose.
