Macworld:

DiskWarrior does one thing, and does it well: it optimizes and repairs disk directories, which contain the information that tells your Mac where files are stored on the disks attached to it. If directories become corrupted, you can lose files. While your data may still be on a disk, the Mac is no longer capable of finding it. DiskWarrior works both as preventive medicine—to fix errors before they become serious—and to correct more serious errors and help recover files when things get really bad.

I am no longer a professional Mac Consultant but, when I was, DiskWarrior was an indispensable tool. I still highly recommend it.