About using storage units that specify a media server
NetBackup lets you create a storage unit without specifying a media server for that storage unit (that is, you can specify for the media server for the storage unit). When a job uses such a storage unit, NetBackup determines the media server to be used with the storage unit when a job runs.
If you specify a destination storage unit that uses media server for Vault duplication, NetBackup may choose a different media server for the duplication job than the source media server. If so, data is sent over the network.
Therefore, to avoid sending duplicates over the network, use storage units that specify a media server (that is, do not use storage units that are configured to use media server).