245 error creating database copy in Exchange 2010 DAG

Here's an annoying one which Microsoft technical support had a hard time with. When you try to create a database copy in a DAG the initial seed fails with a 245 error on the source server. You can work around the problem by dismounting and remounting the database and then telling it to resume replication. The problem is the system essentially tries to start replication before the database is even created on the target server. While Microsoft wasn't able to answer why it's trying to do that, they did give me the correct shell cmdlet to create a database copy with postponsed seeding, which allows you to skip the error and the whole procedure to start initial seeding:

Add-MailboxDatabaseCopy -Identity "Database Name" -MailboxServer "Server Name" -ActivationPreference 2 -SeedingPostponed

That said, even doing it this way you'll might still get an error on the target mailbox server, but it's easier to ignore and don't stop the initial seeding.

The "SeedingPostponed" option is normally used for an existing database, and will create the database copy but leaves replication suspended so you can manually seed it. However, when used for a brand new database the initial log with the database creation command in it is copied over, and that command causes the database copy to be created and begin replication. So the end result is different when using it on a new database like in this scenario.