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In order to enable VSS based backups of Hyper-V VMs using Windows Server Backup, you must add the following registry keys to register the Hyper-V VSS Writer with Windows Server Backup. Note that the WindowsServerBackup key is not created when installing the Windows Server Backup feature. This key must be created manually.

http://blogs.technet.com/askcore/archive/2008/08/20/how-to-enable-windows-server-backup-support-for-the-hyper-v-vss-writer.aspx 

également disponible un article de la base de connaissance : KB958662