It’s important to understand the difference between self-contained movies and reference movies when you use them in your iDVD project.
A “self-contained movie” includes all source files within the movie file, so self-contained movies are usually larger in size.
Movies from iMovie that you have shared (by choosing Share > iDVD, or Share > Media Browser in iMovie) are self-contained movies. You can also create self-contained movies from movies created in iMovie HD and other video editing applications like Final Cut Express and Final Cut Pro by saving or exporting them as QuickTime movies. These QuickTime movies contain all the project files within the movie itself, so after the movie is in iDVD, it doesn’t matter if you change the original file.
A “reference movie” is a movie that points to other source files.
Movies from iMovie HD (those you add from the Media pane in iDVD or those you export to iDVD from iMovie HD using the Share command) are reference movies. iDVD “refers” to the iMovie HD project file to retrieve the movie each time you open your iDVD project. It doesn’t actually re-create the movie in iDVD. So, if you remove clips from the iMovie HD project before you burn your project to DVD, your iDVD project will end up missing those deleted parts of your movie. If you use a reference movie in iDVD, make sure that all references and components of the movie file are located on a hard disk that iDVD can access.
To avoid the problems associated with locating all the pieces of reference movies, convert reference movies to self-contained files instead.
For more information, search for “self-contained movies” in QuickTime Help.