I had trouble using a new LaCie external DVD burner with iDVD 5. Although iDVD 5 claims to work with external burners, the only way I got it to work was to save the DVD as a disc image (under the file menu). Then use Toast to burn the disc image onto a DVD (Disc Utility did not work for burning the DVD). This method worked well.
Optional step: after saving the disk image but before burning the DVD, use DVD Player to play the disc image and make sure it works.
The amazing thing to me was that I spoke to LaCie technical support and they were unable to tell me how to do this. They simply said "We don't support iDVD". For a company that markets to Mac users as heavily as LaCie does, to not know how to use thier burners with the most widely used DVD creation program, I find incredible!
DanK