Or else, you can use the HD Recovery volume that OS X has created when it is installed on a Mac. This may be any drive that has installed a copy of OS X capable of booting. Mac boot drives cannot be unmounted while they are in use, which means you will have to boot your Mac from another bootable device. However, for First Aid to make any repairs, the selected volume must first be unmounted. You can use Disk Utility's First Aid on the Mac boot drive. Before El Capitan, you can only run the verification process yourself and then decide if you want to try it. Now when you run First Aid, Disk Utility will verify the selected drive and if it finds an error, automatically try to fix the problem. The main change is that First Aid no longer has the ability to verify the drive independently of repair. With the advent of OS X El Capitan, Apple made a few changes to how the First Aid feature works in Disk Utility. Disk Utility's First Aid feature can verify the status of the hard drive and, if necessary, perform a repair of the hard drive's data structure to prevent minor problems from becoming a big problem.
