I posted an earlier thread ("Unlocking a CD-RW") in which I expressed frustration with trying to use Retrospect with CD-RW. Many thanks to those who responded.
Turns out that my problem was not with my Combo drive, nor with the CD-RW media, nor with the Retrospect software.
Guess what! It was "operator error." I simply had not designated my media type when I configured my Backup Set in Retrospect's setup screens. DUH!