Quote from the article "Backup Version and Space Management in Windows Server Backup" on Microsoft TechNet:
If the backup storage location is full, Windows Server Backup automatically deletes the oldest backup version to make space for the current backup.
On Windows 7 Professional x64, is it possible to disable this feature, i.e. to tell Wbadmin
to never delete backups?
Instead, if the backup volume is full, I'd like to get an error message.