In contrast with Microsoft Windows-style installers, Apple Macintosh OS X uses disk-image container files known as “DMG” (Disk iMaGe) files to distribute software. Like any other type of file, DMG files can become corrupted under various circumstances, and a corrupted disk image can cause problems on your Mac OS X volume.
When files are transferred from computer to computer via the internet or any other means, errors can occur. Mac OS X uses a checksum-a mathematical operation on the binary data that constitutes the file-to verify that the transferred file is a true copy of the original. A change of even a single byte will change the checksum value. If you get an “invalid checksum” error, the file is most likely irreparably corrupt and will need to be reacquired.
DMG files have two components, which Apple calls “forks.” One is the data fork, which contains data the programs can manipulate, like text in a word processor. The other is the resource fork, which contains the actual program code as well as other aspects of the application, such as graphical elements or sound. Since other operating systems, such Microsoft Windows, don’t use this forked format, transferring to, from, or across non-Mac OS X systems can cause one of the forks to be lost or corrupted.
Attempting DMG Repair with Disk Utility
Mac OS X has a built-in utility which may be able to repair a corrupted DMG file. To access it, enter “disk utility” into the “Spotlight” search box. Then click “Images” on the menu bar and select “Verify,” then select the DMG file in the Finder window that opens. The DMG file will be verified by Disk Utility. Click the name of the DMG file in the left pane to select it, then click the “First Aid” tab, then click “Repair Disk.” You may have to click “Repair Disk” several times to repair all problems.
Replacing Corrupted DMG Files
If the DMG file cannot be repaired, it will have to be replaced. The easiest way to replace a DMG file is to simply delete it and download it again. If the newly-downloaded file is also corrupted, try downloading the file with another browser or to a different disk. If it is corrupted even then, it may be that the file on the application’s website is corrupted and you will need to wait until it’s replaced with a valid DMG file.
Apple: Using Disk Utility to Verify or Repair Disks
The Unofficial Apple Weblog: Mac 101: DMG files are Disk Images