I've only used NTBackup on my XP box at home. At work, we always used enterprise-level backup solutions. (legato, backup exec, Arcserv, etc)
I suggest checking your event logs to see what it's griping about. It could be as simple as the naming of your file. If it's trying to save the same filename, it might not 'overwrite' it.
When you say append, I don't know what that means exactly. I mean, if you're doing a full backup there's nothing to append. Each backup is going to be its own beast. If you're doing incremental or differential, you might be able to append them...