By default, Windows Server Backup (WSB) does not support backing up a volume that contains both active and passive DAG database copies. As you may know, WSB backups for Exchange 2010 are done at the volume level; the entire disk holding your database and its log stream needs to be selected in order to perform a successful Exchange backup. If you have a combination of both active and passive DAG database copies on the same volume, your backup will complete with errors and an Event ID 9782 will be written to the application log:

Exchange VSS Writer (instance 4ad258c8-4b21-4236-94e3-4582747f062e:1) has completed the backup of database ‘DB1’ with errors. The backup did not complete successfully, and no log files were truncated for this database.

Fortunately, there is a setting that can be changed to allow you to perform successful backups when a volume contains both active and passive databases. This is accomplished by disabling the Exchange Replication Writer via the registry, forcing the Store Writer to be used instead. This will ensure that you can get a good backup of the active database and the passive database will be ignored. To enable this setting, perform the following steps:

1. Navigate to HKEY_LOCAL_MACHINE\Software\Microsoft\ExchangeServer\v14\Replay\Parameters
2. Add a new DWORD value named EnableVSSWriter, and set its value to 0, as shown below:

3. Exit Registry Editor and then restart the Microsoft Exchange Replication service.

At this point, you can run your backup job on the volume that contains the active and passive Exchange databases. Once completed, the status should indicate that the job finished successfully and an Event ID 9827 will be written to the application log:

Exchange VSS Writer (instance 23e95ab1-bd0f-43de-b3f8-f437c36f5b05:2) has successfully completed the full or incremental backup of replicated database ‘DB1’. The log files will be truncated after they have been replayed.

For more details on backing up Exchange 2010 with WSB, check out my previous post, and also see Understanding Backup, Restore and Disaster Recovery on TechNet.