You can count on one thing: If you’ve moved files around, deleted programs and applications, and emptied 5 GB worth of trash, your hard drive will be a mess internally. And yes, I know the Mac OS X automatically defragments some of these files automatically, but it does not “optimize” the files when it’s done. To get the most from your hard drive, I think a third party defragmentation and optimization program is essential.
Here’s basically how a hard drive works. A hard drive is a circular disk, kind of like an LP record. As data is saved to the disk, the disk spins and the data is written sequentially, starting with the first open space it finds. If that space isn’t big enough, the rest of the files’ data is stored somewhere else. As you install and uninstall programs and add and delete files, the holes for data on the hard disk become plentiful and you end up with files stored everywhere on the drive. When you open a file that is stored on the hard disk in several different places, the disk has to spin many more times to collect the data and put the pieces together. The more the disk spins, the more wear and tear on your machine, which causes problems and degrades the performance of your hard drive.
When this happens, the computer’s response time slows down because it simply takes longer to obtain and organize the data than it would if the files were stored contiguously. The problem is worse if program files are stored in a noncontiguous manner, as you can imagine. After a computer is defragmented, the files are stored (more) contiguously and the computer’s hard disk has to spend less time spinning around and looking for files. This makes for better performance and less stress on the computer.