ntbackup.exe does not allow backing up directly to CD.
From X:\VALUEADD\MSFT\NTBACKUP\README.TXT
[[NTBackup Backup/Restore and ASR release notes
2. Backup to CDRW
If you decide to backup to a CDRW, you cannot target that device directly.
You must create a backup set of 650MB or less and backup to a file. After
the file is complete, copy the file to the CDRW.]]
you can use ntbackup to backup on remavable media like CD/Floppies this task
is handled there by a service called removable storage check to see that it
is not disabled, when it is not disabled you can store your backup onCdrw