From Alsoft Inc.:
DiskWarrior is a utility program designed from the ground up with a totally different approach to preventing and resolving directory damage which is the leading cause of Mac instability.
DiskWarrior resolves existing disk damage by rebuilding your disk directory using data recovered from the original directory, thereby recovering files and folders that you thought were lost. The directories DiskWarrior creates are also optimized for maximum directory performance.
DiskWarrior is not a disk repair program in the conventional sense. It does not attempt to solve all of the possible problems that can occur with a disk. It specializes in eliminating directory errors - the most common problems Mac OS users have with their disks.
DiskWarrior rebuilds your disk directories making them error-free, eliminating any problems the errors would have caused, and recovering lost files.
It fixes any problem with master directory blocks and alternate master directory blocks (HFS), volume headers and alternate volume headers (HFS Plus), volume bitmaps, catalog trees, and extents trees.
This approach has a significant advantage that the others overlook. Other products attempt to repair your directory one step at a time, ignoring the big picture of what is truly wrong with your directory, and risking file information that could be saved. Too often, they will incorrectly begin to "repair" your directory, and then give up when they realize that they have modified the directory to an irreparable state. At Alsoft, we know that the most important thing when you are attempting to repair your disks is getting your data back. That's why the first thing DiskWarrior checks is your directory data, and that's what it uses to create a replacement directory. DiskWarrior continues its rebuild operations unless no directory data can be found.
Directory damage isn't the only threat to your data. A physical hardware malfunction can cause intermittent loss of access to any data on the drive. If the malfunction is serious enough, the only way to retrieve the data on the drive is through an expensive data recovery service. DiskWarrior can be configured to automatically check drives for possible drive malfunction, giving you the chance to back up your data before a drive completely fails. The hardware monitoring can also be run manually if you suspect a drive malfunction.
Alsoft has been making disk utilities for the Mac over 25 years. Our disk optimization utilities have always chosen the safety and security of your data over anything else, and have been able to detect and report directory errors no other program can (the others obviously couldn't fix the problems they couldn't even find). That knowledge has been put into DiskWarrior to give you the only tool that can eliminate all directory problems and recover files and folders! And, as an added bonus, the directory DiskWarrior creates is optimized for maximum speed when accessing your files!