P erhapsthe most frequently used tool in Disk Utility’s arsenal is Disk First Aid. It scans whichever drive you select for problems in the structure and integrity of the drive. If you select the whole drive, rather than a single volume, it checks the partition maps for the drive and the directory, then checks each volume. If you only select one volume, it just checks that. Volumes appear under the main drive in Disk Utility’s sidebar. For example, the internal drive on your Mac contains a Macintosh HD volume which, in turn, has volumes for data and for system information.