Backup - Incorrect Function (0x80070001)

I'm trying to backup my system onto DVD-R discs. It runs for about 15
minutes and then gets this error message: "Incorrect Function (0x80070001).
If I say try again, it happens again after about the same amount of time.
I've rebooted, but that doesn't help. It appears to happen when the first
DVD is full. I'm running Windows Vista Home Premium on a Dell XPS410.

I backed up the system onto a Western Digital external hard drive yesterday
with no problems. I now want to make a copy I can take off-site, but I can't
get it to run. I think it took about 50 GB to do yesterday's backup.
--
Lyle
Montrose, Colorado