Your Subject line refers to 'backing up computer' while
the error you received deals with 'recovery' disks.
Recovery disks are not backup disks.
XP will let you make one (and only one) set of recovery
disks. These are used to recover from operating system
crashes and problems.
Backup disks are what you make daily/weekly or not often
enough. This realization usually arrives right after you
develop the need for a recovery disk.
So - perhaps you're getting this message because you've
already made your allowed single set of recovery disks???
OR
I received a similar error once and it turned out that I
was using a CD-ROM disc rather than a CD+ROM disc (one is
a minus and the other a plus).
Bill