iDVD external burners
iDVD 4.0 does not allow you to burn a DVD using an external DVD Burner. However, a hack exsists that unlocks a hidden menu that allows iDVD 4.0 to recognize external burners. It also allows you to create an image file of the DVD.
With an image file you could also use Toast to burn a DVD on an external DVD burner.
For those of you who have upgraded to iLife 5.0, the new iDVD 5.0 does allow you to create disk images of your DVD, without having to utilize a hack.
