To back up a Windows client with the VSS method, please note the following about snapshot parameter values:
If you selected a of 3 (hardware) and a of 2 (plex): You must configure an appropriate number of clones or mirrors in the disk array, depending on the value that is specified for the parameter. If the Maximum Snapshots parameter is set to 3, you must configure three clones or mirrors. If you do not want to use Instant Recovery and the Maximum Snapshots parameter is 1, you need only configure one clone or mirror.
You must also synchronize the clones or mirrors with the disk array source device before starting the backup. If the clones or mirrors are not synchronized before the backup begins, VSS cannot select a clone or mirror on which to create the snapshot. The backup fails.