Disk Utility allows you to work with your hard drives. You can use Disk Utility to format, erase, partition a drive and also to repair problems that may crop up from time to time.

Disk Utility provides two unique functions. One lets you repair file and folder permissions and the other help you repair a hard drive.
Repair Disk

Disk Utility repair common disk issues like corrupt directory entries to files left in unknown states, usually from power outages, forced application quits, or forced restarts . Disk Utility’s Repair Disk feature is extremely good at making minor disk repairs to a hard drive’s file system, and it also make most repairs to a hard drive’s directory structure, but it’s no replacement for a good backup planning. The Repair Disk feature is not as determined as some third-party applications that do a better job of repairing hard drives as well as recovering files, something Repair Disk is not designed to do.
Repair Disk Permissions

Disk Utility’s Repair Disk Permissions feature is designed to restore file or folder permissions to the state the OS and applications expect them to be. Permissions are flags set for each item in the file system. They define whether an item can be read, written to, or executed. Permissions are first set when an application or group of files are installed. The installation includes a .bom file that lists all the files that were installed, and what their permissions should be set to.

Using Disk Utility to Repair Hard Drives and Disk Permissions (Disk Utility).

What You Need


Disk Utility, located at /Applications/Utilities/.
Ten minutes to an hour of your time, depending on which First Aid tools you will be using.

Disk Utility’s Repair Disk feature works with any hard drive connected to your Mac, except the startup disk. If you select the startup disk, the ‘Repair Disk’ button will be dulled out. You will only be able to use the Verify Disk feature, which can examine the drive and determine if something is wrong.

Repairing a startup drive with Disk Utility is very possible. To perform this operation, you must boot from the OS X installation DVD or boot from another hard drive that has OS X installed. Aside from the time required to restart from another hard drive an installation DVD or the Recovery HD, using Disk Utility’s Repair Disk feature otherwise works the same way and takes the same amount of time.

Repair Disk

1. Back up your drive first.

It is always a good idea to create a new backup of a suspect drive before running Repair Disk. While Repair Disk usually doesn’t cause any new problems, it’s possible for the drive to become unusable after an attempt to repair it. It’s just that the drive was in such bad shape to begin with that Repair Disk’s attempt to scan and repair it kicked the drive over the edge.

2. Launch Disk Utility (located at /Applications/Utilities/).

3. Select the ‘First Aid’ tab.

4. Select the hard drive or volume you wish to run Repair Disk on, in the left-hand pane.

5. Tick in the ‘Show details’ box.

6. Click on ‘Repair Disk’ button.

7. If Disk Utility encounter any errors, repeat the Repair Disk process until Disk Utility reports ‘The volume Bleep appears to be ok.’

Read more: http://trendymediablog.com/2014/08/using-disk-utility-to-repair-hard-drives-and-disk-permissions/