The directory of your Mac OS X needs to stay away from damages and corruption for healthy working of your Mac. If the directories get damaged, the respective Mac volume comes to an inconsistent state and becomes inaccessible. This, in turn, makes the data inaccessible and further aggravates to permanent disk failure.
Stellar Volume Optimizer is the ultimate tool for Mac users that verifies and repairs almost all volume directory damage or corruption. It works both for the boot volume and for the non-boot volume.