When I do back ups to CD, I use CD-Rs, and if I use the Windows CD Writing Wizard, I can add to the disc at a later stage, so it obviously doesn't finalise when it writes data, like it does when it writes a music CD. It's the same as creating a multi-session disc in Nero, if I understand it correctly. Try using the Windows CD Writer program next time, Lona, and just with a CD-R.