With Microsoft Virtual Server 2005, it was impossible to back up running VMs with the built in NTBackup utility. Instead, Virtual Server 2005 SP1 introduces a VSS Writer that backed up virtual disks. However, NTBackup was still not supported in Virtual Server 2005.

With the introduction of Windows Server 2008 Hyper-V, the NTBackup limitation is gone. However, the removal of the NTBackup limitation is not something that is automatically available. There is a trick to get NTBackup to work with Hyper-V virtual machines. You have to tweak the registry to register the Microsoft Hyper-V VSS writer with Windows Server backup. Note that there are some backup/restore limitations referenced in the Microsoft Support article. Virtualization.info gives us some more potential limitations including:

• WSB only supports volume based backups: if the VM configuration file and the actual virtual hard drive (.VHD) are stored in different volumes, all volumes must be selected.
  Conversely, when performing a recovery from backup, the entire volume or volumes must be restored.
• Live backup is not supported for those VMs that have dynamic disks. In such case only offline backups can be performed.
• If the VM has more than one snapshot the restore will fail
• Live backup is unavailable for those guest OSes that don’t support VSS, like Windows 2000 or XP, as well as those guest OSes that don’t have the Hyper-V Integration Services installed. In such cases WSB will put the VM in a saved state, backup the snapshot, and finally restore the VM.

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