I think eiher I misunderstood you or you misunderstoord me:
Buffer underruns can only happen when recording the image, not when writing the image to harddisc! Like I said your harddisc is too slow to write down all data into an image but that's not a problem. It will take a bit longer that's all. When writing an image to harddisc, data is going much faster when writing the image to a CD-R. For example:
MAKING AN IMAGE: can be up to 32X (5MB+ per second)
WRITING THE IMAGE: can be up to 8X (1.5MB per second)
You see? Your harddisc has to work much harder when making the image so that's why you het a full buffer when making the image. Writing the image from harddisc to CD-Writer is a far lesser strain on the harddisc.
A buffer underrun is not likely to happen when writing at 8X. I hope I've explained it