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.
No it is not possible. Windows Server backup is a simple, free backup tool that works well enough, but lacks the flexible, robust, granular features of an enterprise-grade backup solution. WSB monitors the free space left on the drive and if the drive is full, it deletes the oldest backup before taking a new one. And that's all you get. If, for some reason, you want backups to start failing when the drive is full instead of taking another backup, then you'll need a more robust backup application that allows you to make that choice.
