Quote Originally Posted by Arche
Maybe the image is corrupt. It's a dreamcast CD, not a PC CD? I wouldn't be surprised if it couldn't read it then.
Images you download for the DC are PC images. The only particularity is the boot loader they have (to be selfbootable on the DC), and Nero, unless the image is corrupt, shouldn't have any trouble reading it.
.cdi is diskjuggler, so look for that program.