what would be the best way to go? Insert the valid disc and copy it to my desktop as a disk image and remove it and burn an image from that using Disk Utility?
Yes. That probably would be the easiest way to go.
Have you edited any of the source material since burning the first disk? If so that brings up a know iDVD problem that requires you to start over from scratch with a new project file and add the media files to it again. The iMovie file that you added to the original project should be on your hard drive.
Give this a try to open the current project file:
1 - delete the IDVD preference file, com.apple.iDVD.plist, that resides in your
User/Home/Library/ Preferences folder.
2 - delete IDVD'S cache file, Cache.db, that is located in your
User/Home/Library/Caches/com.apple.iDVD folder if there is one.
3 - launch IDVD.