Hard drives are expensive, and the data on them even more so. When my laptop was hit by a trojan, it took me a week to reinstall everything. That cost me more in lost time than the cost of the entire laptop!
By comparison with the speed of RAM and the speed of the processor, the hard drive is the slowest part of most systems (excluding peripherals). So if you can get your hard drive to perform well, your system performs well. One of the factors that causes a drive to overwork is the mis-placement of files on the drive, usually caused by file fragmentation, but not always.
Another factor relating to hard drive performance is the way the hard drive works. The data on the "outside" of the drive can be accessed faster than the data closer to the spindle. The blue line in the graph below shows data transfer rates, and the yellow spots show seek times, with the "outside" of the drive on the left, and the "inside" in the right. Notice how the speeds change from 33MB/sec to 18MB/sec, with the average around 23MB/sec. So a file placed near the "start" of the drive (the outside) can be accessed and read quicker than the files near the "end" of the drive. Some defrag utilities (e.g. UltimateDefrag, JkDefrag, PerfectDisk) take this into account when deciding where to place the files.
As the drive works, its temperature rises, especially in the confined space of a laptop. Utilities like HDTune (free) can warn you if your drive is getting too hot. Hard drive utilities like SpinRite (US$89) will even stop operations and warn you if the drive is overheating.
The Paragon Total Defrag 2007 (US$29.95) help file has an excellent summary of disk defragmentation, 
with diagrams.
DiskTrix UltimateDefrag (US$39.95) has a help file that explains the theory in more detail. I have included some of the pages, and you'll get the entire file when you download a trial version of UltimateDefrag.
Here are extracts from my own article on why you need to defrag your hard drive:
Defragmentation isn't the only strategy to make a PC run faster, but it is one of the strategies that are useful. "Regular" defragmentation is better than "random" defragging, but how often is "regular"? It depends on your PC. If you are generating lots of files, or editing databases, a daily defrag may be required. If your files are pretty small, a monthly defrag is probably good enough.
My own rule of thumb: if the defrag takes longer than an hour you need to do it more often. If it takes less than 10 minutes, defrag less often. You don't want to waste a lot of time in order to save a little time. Do the defrag during a quiet period, such as when you're away from your desk for a meeting, or after hours. In that way you don't waste any productive time.
Update: This page will be retired and moved to http://www.fact-reviews.com in the near future.
available for download here (version 3.26).
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All information copyright © 2005-2010 Black and White Inc. All rights reserved. First published 16 June 2007. Last Updated: 18-Jul-2010 16:45
