How can I make a copy of a dvd made with imovie?
My presumption is that when you burned the DVD using iMovie - it was encoded within a VIDEO_TS folder and you will need some software to move the video and sound files (which are hidden and obscured on the filesystem) back into a format that can be played on your Mac and/or burned back to a fresh optical drive.

Depending on the version of software that wrote the DVD to disc, you might be able to locate the raw video files (or find them on a backup). Barring that, using Disk Utility to simply clone the volume onto a new drive isn't a good choice, but I've had great luck with programs like the little app factory's RipIt.