Are you seeing any VSS/Snapshot provider errors in the Event Viewer within the guest OS?
824 warning 'task-2'] [UsedClusterBitmapImpl] FlushFileBuffers failed for volume \\.\GLOBALROOT\Device\HarddiskVolumeShadowCopy7, error: 19
2013-02-19T05:50:56.734-08:00 [02824 warning 'task-2'] FileClusterBitmapBuilder [AddBadBlockExtents] Unable to get bad blocks extents for \\.\GLOBALROOT\Device\HarddiskVolumeShadowCopy7\. Error: 5
Regards
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