My gut feeling, from having worked extensively with tapes many years ago on IBM mainframes which didn't yet have disk drives, is that you've got an unreadable end-of-data marker on the first tape Member of the input Backup Set. The computer operators—a separate profession in those days—used to have a utility program that could write an end-of-tape marker on a tape, but AFAIK we don't have such a program now. (Some folks may: In 1992 I had backed up my wife's Mac using a Maynard QIC tape drive before I erased her HDD, and the Maynstream backup program—the ancestor of BE—couldn't read the tape to restore it. I paid DriveSavers US$600 to recover the data from the tape, which they did; it'd now cost around US$2000.)
If you've got the data from that input tape on the output Backup Set, IMHO what you should do is to run another Transfer to the same output Backup Set using only the remaining tape Members of the input Backup Set—starting with the 2nd one. A way to do that would be to mark the first tape Member of the input Backup Set as Missing, per the NOTE under "The Members Tab" starting at the bottom of the left-hand column on page 155 of the Retrospect Mac 6 User's Guide.