When snapping a 2008 R2 Standard / Enterprise Hyper-V Server that has Guest VM VHD's on a protected disk the snapshot of the host will fail with the following: "Hyper-V Event Viewer Error"
HYPERV EVENTS / EVENT 10111, Hyper-V-VMMS
Failed to mount the virtual hard disk \\GLOBALROOT\Device\HardidskVolumeShadowCopy5\Data Virtual Hard_disk 4dfj68-3435n-df45kj6-8735798u7b.avhd for virtual machine "EAMSDBTEST" (Virtual machine ID 21135FJ6JG-0DFC4-34978-DRG-34658ES) .
The VSS writer 'Microsoft Hyper-V VSS Writer' failed during snapshot phase 'DoSnapshotSet' with error 2147754806. Writer state is WaitingForBackupComplete
If there are Guest VM's running on the Hyper-V Host being backed up, when the snapshot runs the host must have the ability to auto-mount or you will receive Hyper-V and Application Event Errors