The following post is a simplified explaination of some of the more common and important specs a person needs to know when comparing hard drives. For the sake of this post, I will only cover ATA/SATA hard drives as they are the most common for home users. To understand what the specs mean, you first need to understand how a hard drive works. When the computer’s hard drive has to be accessed… the drive first has to be searched before anything can be sent and when the information is located, the CPU (computer's brain) gets the information only as fast as the disk drive can physically spit it out. The slower the drive, the slower the data comes from the drive. Average Seek Time: This is the average time (in milli-seconds) required to search a hard drive for the data it needs to send. The lower the number, the less time is typically required to search before the drive can begin sending data to the CPU. Rotational Speed: This is the physical speed at which the hard drive rotates. Most ATA drives rotate at either 5400 or 7200 RPM. * The advantage of a higher rotational speed is that the drive can send the data faster. * The benefit of a lower speed drive is less noise and often less cost. So for example: A standard 7200 RPM hard drive sends data significantly faster than a 5400 rpm, but much slower than a 10,000 RPM drive. The 5400 RPM drive would be the quietest (good for music studios where noise is important) whereas the 10000 RPM would be the loudest. Cache/Buffer: The cache is a RAM type memory block, used by the hard drive, that stores the drive's "predicted" data selection based your last choice. For example: If you open a file in Word called "Stamps", the drive may also store in it's cache another word document you have stored on it called "Post Office". If you open the file "Post Office" next, instead of the drive being searched, and then retrieving the data at the speed of the physical drive, the drive's RAM cache sends the already stored "Post Office" to your computer's RAM. This means that instead of it taking several seconds to search, retrieve and load the data, you'd have it instantly. In theory, the higher the cache, the more likely you can have this instantaneous RAM to RAM situation occur. In reality, it seldom happens that way. Parallel or Serial ATA Interface: These are the two main drive styles used by home users. Parallel ATA is also known as Enhanced IDE (EIDE). Parallel (PATA) works on the premise that you add more channels to increase the speed the drive sends data to the computer. So, the data cable for PATA, which is usually about 1 inch wide and has a maximum length of 18 inches, connects using 40 pins. Most manufacturer's will recommend using an 80 wire (still 40 pin connection) EIDE cables because the 40 extra wires are twisted grounds that reduces BUS (background) noise and make the data transfer faster and better. This style of drive has been used for years. It has a maximum output of 133MB. However, unless you are in a RAID or unless you're cache has accurately predicted your next request, you will never hit that speed for output. For most 7200 RPM drives, the average output is approximately 50-60MB. By contrast Serial ATA (SATA) is a relatively new technology. Unlike the PATA, the data cable for the SATA is very small, about 1/4 inch wide. Where PATA sends data down multiple "channels" at the same time, SATA has only one channel and it send the data one bit at a time. The best analogy I can explain this is like a hose to water your lawn. PATA is the open hose. The water is coming out pretty fast but SATA is like when you put your thumb over 3/4 of the opening and force it to out faster and more as a direct spray. Because of this method, SATA actually has a higher maximum output and the maximum cable length can also be one meter (39 inches). But again, like the PATA, unless you are using the drive in a RAID or the cache has the data already, you will never hit the max output of the SATA drive. One final note on PATA/SATA: the Operating System treats them the same way. Newer OS'es will know the difference between them but older OS'es wont and both will work assuming you have the right physical connections.