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In my case, I have automatic failover. So one day, server1 is the principal and maybe after windows update or something else, server2 may be the principal. Even if I create a script which forces a fallback to server1. If server1 is down for some reason, I will not have any backup or i have to manually change the backup jobs. So there is a chance that I haven't a backup for a few days if no one is there to change the backup jobs after a failover.
The best way would be the ability to specify principal and mirror and backup exec could decide which is online and should be backedup.
The other way would be an option to allow backup exec to ignore the mirrored database. So i can select it, but wouldn't get an exception if it is mirrored.