I had a problem with backing up to DVD RW disks, after inserting a new blank disk, backup would say "the drive is in use by another program". Turns out, the "Autoplay" function would activate when a new disk was inserted in the drive, and Autoplay was interfering with Backup. Workaround was to switch off the autoplay function for that drive.
I successfuly switched off autoplay, and now backup works fine. Odd thing is, now I can't remember how I did it, and I could not find any info on how to switch off Autoplay!.