Maimon

I try to read a multissesion, not finalized CD-R burn with Nero, to copy it. The following message appears: "Disc Not Supported (Multi-session)". But if I try to read a DVD+R, ISO+UDF, multissesion, not finalized disc burn with Nero, no error message appears. I can read the disc. My questions are: If the read is a "sector-by-sector" operation, I would expect to be able to read any multissesion disc. Why I can read a multissesion DVD+R, but not a multissesion CD-R?