Q: How do I burn a dvd to another dvd on my mac desktop

How to copy previously-burned DVD-R video discs

http://support.apple.com/kb/HT2059

To create an image file, launch Disk Utility and select File: New: Disk Image From Folder. In the window that appears, navigate to the desired folder and then click on Image. Give the image file a name, and click on Save when prompted. That’s it.

Easier still: use Roxio Toast.