Tivoli Storage Manager for Mail is used to back up an exchange database in a database availability group (DAG) environment. This document explains which vss writers are used.
Diagnosing the problem
The Exchange VSS writers document from Microsoft mentions the following :
"Of the two Writers Exchange Server 2010 provides, one is built into the Exchange store, and is referred to as the Store Writer; the second one is built into the Replication Service, and is referred to as the Replication Writer. The Store Writer is available on any mailbox server and is responsible for backup/restore of active databases. The Replication Writer is available on the passive node of DAG server. The Replication Writer supports backup functionality for a selected database where the shadow copy is taken against the replicated instance of the database and transaction log files."
Resolving the problem
This means that when Tivoli Storage Manager for Mail backs up an exchange database, if the active database is backed up, the "Store Writer" is used and if the passive database is backed up, the "Replication Writer" is used.