Sometimes OS X gets its indexes in a twist and the built in Disk Utility just cannot get the directory structure organised correctly. When this happens, you're given a message that you should back up your data immediately and re-format your drive. Sometimes a serious error will result in your files system being shut down into Read Only mode. If this happens you will not be able to boot your Mac.
We have advanced data recovery software built for Apple Mac OS HFS+ that can do what Disk Utility and other software cannot. It will scan over the entire drive and rebuild your b-tree directory and file system and mount a read-only recovery drive for proper data recovery even on drives with bad sectors.
For less than critical errors, a Disk Warrior service can sort out your indexes, reconstruct them correctly and write the optimised indexes to disk resulting in a working volume again. This is a much better option than re-formatting plus data backup / recovery and will save you time and money.
Disk Warrior will also audit your plist files for errors and repair disk permissions to ensure maximum performance from your Mac.