How to Repair DMG Filesby Theon Weber
DMG files are Apple disk images--single files that represent the entire contents of a physical disk and can be "mounted" on a computer to provide access to that contents. Just like real disks, they can become corrupted and damaged, and are often difficult to fix. However, Apple's built-in disk utility may help you repair minor damage to a DMG file.
Open "Applications," then "Utilities" and double-click on "Disk Utility."
Drag the DMG file into the disk utility's left pane.
Select the DMG file you dragged in, click the "First Aid" tab, and click "Repair Disk." Follow the on-screen instructions.