More info, looks very relevant . . .
Exchange 2010 involves 3 physical servers - the Mailbox databases are spread between them. The Replications that are working are just the Mailbox databases that are Active on *one* of the Exchange servers. If the Mailbox db is active on one of the other 2 servers, replication of that mailbox isn't working.
Does that point to a problem with the DP9 Backup (some setting in the Specification that we've missed, that forces all servers in the DAG to take part)? In the specification, the source is the whole DAG; the policy settings are Backup Passive Copy, with policy to Minimise No of Hosts, and Use Active Copy if no Passive Copies are available.
Any further ideas?
By the way, everything is Windows (except the StoreOnce 4700's); and the replication of the Weekly Full seems immune to this problem - it is only the replication of daily Differentials that seems affected.
Thanks
