When you copy files to an empty hard drive, the data is written to contiguous (adjacent) sectors on each disk; think of playing cards being laid end to end. This makes it easy for the Mac to retrieve the information later because every piece of data is grouped with the data that's supposed to come next. That's why a brand new drive is the fastest drive.

But things don't usually stay that way. Over time, drive space gets used up. Small files get deleted, leaving little pockets of available space here and there. File fragmentation occurs when, after extended use, the only storage space available on your disk is in various unconnected sectors, scattered in various locations. At that point, the Mac begins to save files in pieces, wherever empty sectors are available.

The Mac can still access such fragmented files. However, reading fragmented files takes longer because the hard drive's read/write heads must physically move around to find all the pieces of the file.

You can regain the tiny bit of speed lost through file fragmentation by defragmenting your hard drive. Defragmenting also increases the chance that you'll be able to recover files after a crash by using a repair utility; your files will be in solid chunks and not scattered across the drive in tiny segments.

To defragment (or "defrag" as the geeks would say) a hard drive, you can either buy a disk-defragmentation program (such as Norton Utilities) or simply reinitialize your hard drive (a task you can easily accomplish with the utilities that come with your Mac OS). Reinitializing a hard drive erases all the data on the drive, however, including the operating system and all your applications. Make sure you back up your drive completely before reinitializing, and keep in mind that you'll have to reinstall your system and applications — which is why it may be better to invest in a defrag program, so you don't have to reinstall everything.