"The currently inserted disk is a DVD which can only be written in one session. Would you like to erase all existing data before writing? - Yes or No" appear and what does it actually mean??
It would mean that the first or last write operation done on the disc was written in such a fashion as to prevent appending of data. It was not a Multisession disc and so the disc could not be reused as it was.
The software and recorder detected that you had a ReWriteable disc in the recorder, and so offered you the option to 'erase all existing data before writing'. This would have caused the disc to become blank and therfore writable again.
Our Nero BackItUp utility is designed to span CD and DVD Media of all types, and to allow backups on multisession CD or DVD media till the disc is full.
If you use our Hard Drive Backup option, this does a sector by sector backup of the selected partition or hard drive. It spans CD and DVD Media as well, but does not allow multisession. This creates a bootable disaster recovery, system and all.
PS. Make sure you have our latest Nero 6 software installed. This will ensure that you have our latest BackItUp version.