Will iDVD support an external drive? I'm thinking it doesn't.
You might want to check the drive database at
www.xlr8yourmac.com and see which drives are supported by the iApps (particularly iDVD). I'm fairly certain that DVD Studio Pro supports external drives.
It may be to your benefit to install the "superdrive" as an internal drive, and move one of the others to an external case (instead of having the DVD burner as an external drive).
I'd recommend one of the Pioneer drives (DVR-106D), as these are the mechanisms that Apple frequently uses. For Macs, definitely go "-" instead of "+". In addition, DVD-R is more compatible with stand-alone DVD players than DVD+R is.