[jabohn]

This problem seems to have begun after I applied the Mac OS 10.4.5 Updater... when I select a Disc Image to make a copy, Toast says the file doesn't appear to be a valid Disc Image. I can click to verify that it is a valid file, but when I click MOUNT it comes up as an audio CD. If I choose not to verify that it is a valid image, then it says it is "not a valid DVD video image". The file is neither and audio nor a DVD image, it is merely a disc image.

The disc image opens properly with Apple's tools.

Sigh, forget it. I solved the problem.
DUH.