Starting to get somewhere with this guys.
I have a 4 node DAG - 2 in Peoduction, 2 in DR. One of the DR nodes is a lagged copy.
I ran Exchange SP2 on each box with intermittent results on each box. Sometime I had to try 3 or 4 times to get the install done. I think the lagged copy server has suffered from a partial upgrade of SP2 which has affected the Exchange install. The SA service won't start. I removed the lagged DB copies from the DAG then removed that server and I don't get the issues I was having previously with SME.
I do however have an issue with trying to schedule a backup across the nodes now with many errors appearing when atetmpting to create a task