Normal backups copy the files and mark each file as having been backed up. Basically it leaves evidence behind of the backup process so that you don't waste time backing up files that don't need to be backed up.
A copy backup copies the files but d...
http://fixunix.com/veritas-net-backup/481562-catalog-backup-fails-media-manager-system-error-occurred-174-a.html
It's a couple years old, but it sounds like it might be the same problem. Post #4 has the solution that worked for the question poster....