This article is from the Introductory Macintosh FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
As disks fill up it gets harder and harder to find enough
contiguous free space to write large files. Therefore the
operating system will often split larger files into pieces to
be stored in different places on your hard disk. As files
become more and more fragmented performance can degrade.
There are several ways to defragment a hard disk.
The most tedious but cheapest method is to backup all your
files, erase the hard disk (and you might as well reformat while
you're at it. See question 4.10.), and restore all the files.
A number of payware utilities including Norton Utilities
and Mac Tools can defragment a disk in place, i.e. without
erasing it. Although the ads for all these products brag about
their safety, once you've bought the software and opened the
shrink-wrap they all warn you to back up your disk before
defragmenting it in case something does go wrong. If you use
any of these products, be sure to run a disk repair package
on the disk you wish to defragment before defragmenting.
Defragmenting will almost certainly make any existing problems
with a disk worse so it's important to make sure a disk is in
good health before using a defragmenting utility on it. Fast
Unfrag is a $10 shareware disk defragmenter by Kas Thomas. See
It appears to do the job it was designed for; (defragmenting the
files on a hard disk) and my brief tests didn't reveal any glaring
bugs or trash any files. Nonetheless, I'm a bit nervous about this
product because the programmer and his skill level are unknown to
me, and writing a disk defragmenter is not something I'd trust to a
novice. The interface is flaky; the program only works on the disk
where the application resides (very unusual behavior for a disk
defragmenter); it's unfriendly to background applications (not so
unusual for any disk intensive app); and neither documentation,
online help, nor an email address are provided with the program.
Since this is still a relative unknown I *STRONGLY* recommend that
you backup your files before using it. I'd appreciate hearing any
experiences you have with it.