Choose a name and a destination from the “Save As…” window. Keep in mind that your destination should have enough space to copy the entire DVD, which can be over four gigs.
In the “Save As…” window, be sure the “Image Format” selection is set to “DVD/CD Master” and the “Encryption” selection is set to “none”. Click Save.
After copying the DVD to your destination drive, return to Disk Utility. Your new Disk Image will appear in the list on the left of the Disk Utility window. Eject the original DVD disc from your Mac.
Insert a writable DVD disc into your Mac. Select your newly created Disk Image from the list on the left of the Disk Utility window. Click on “Burn” in the Disk Utility main window. Select your burn speed and whether you would like to Verify Burned Data.
You will receive confirmation once you DVD is successfully burned and you are all set with a fresh copy of your DVD disc. Simply save your new Disk Image and you can make a copy whenever you may need it.