A: Backing up a running database requires a process called quiescence. This "quieting" of the database creates a point in time from where a backup job can begin its activities. That point in time allows the database to continue operating during the backup.

A Hyper-V Virtual Hard Disk (VHD) file is much like a database in that changes are constantly occurring inside the virtual machine. Thus, backing up a VHD from the host requires the same quiescence process. This process is enabled through the Microsoft Volume Shadow Copy Service, and requires a specific VSS writer for Hyper-V.

The Hyper-V VSS writer is installed with the installation of the Hyper-V role, but it isn't automatically registered with VSS. Registering the writer requires navigating to HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion, adding the keys WindowsServerBackup\ApplicationSupport\{66841CD4-6DED-4F4B-8F17-FD23F8DDC3DE}, and then adding a REG_SZ value of Application Identifier with the value of Hyper-V.

You can then verify that the Hyper-V VSS Writer has been registered by entering the command

vssadmin list writers

You should see the Microsoft Hyper-V VSS writer in the resulting list.