What occurs if a system-state snapshot is taken while a streaming-style backup is occurring?
If a streaming-style backup operation is in progress, and the VSS attempts to create a shadow copy, the streaming backup will fail and the shadow copy will also (likely) fail. Remember that a shadow copy of an Exchange Server 2003 storage group is triggered when the VSS is used to back up the system state of a disk that contains an Exchange Server 2003 storage group.