If you’re having problems with your hard drive or need to make changes to it, Disk Utility is a good place to start. Start by clicking a disk or volume in the column on the left and then click one of the five tabs described in the following sections.
First Aid tab
If you suspect that something’s not quite right with your Mac, the First Aid portion of Disk Utility should be among your first stops. Use First Aid to verify and (if necessary) repair an ailing drive. To use it, click the First Aid button on the left side of the Disk Utility window. Click a volume’s icon, and then click Verify. You get information about any problems that the software finds. If First Aid doesn’t find any problems, you can go on your merry way, secure in the knowledge that your Mac is A-okay. If verification turns up trouble, click Repair to have the problem fixed. You can also use First Aid to fix disk-permission problems.