When the backup is at 49%, the backup device gets initialized and starts working. Everything before 49% has nothing to do with the actual burner and is just OneCare preparing the files for backup.
If the process fails at 49% it's usually just a bad CD/DVD or could potentially be another software that has locked the burning device and OneCare cannot access it.
I would chock it up to "general bad CD/DVD media issues" as it sounds pretty intermittent. I know personally that whenever I burn something I usually give it about a 80% chance of success, which is the same success rate that I normally see in all burning software that I have tried.
If OneCare backup was broken, it would not burn at all. When the problem is intermittent like that it's usually cheap CD/DVD media problems that's causing the issue.
I hope this helps
