How Do I Repair A Disk In OS X Mountain Lion?
Open your Applications folder, then Utilities, and launch Disk Utility. Select the hard drive in the sidebar, click on the First Aid tab, and then click Verify Disk. If it says you need to repair disk, then click Repair Disk.
If you have started up using the drive that needs to be repaired, you will have to restart to Mountain Lion Recovery. Restart and hold Command-R until the Apple icon appears. Then go to Mac OS X Utilities > Disk Utility and repeat the steps from the first paragraph.