store all finalized projects on an external HD,
Are you referring to the iDVD project file or a disk image of the iDVD project?
If it's the project file it could be that the links to the source material have been broken. Try using the File ➙ Archive Project menu option and put the archived project file on the EHD. That copies all of the source material into the project package so the file can be moved without breaking links.