Before performing any type of software or hardware maintenance on a DAG member, you should first place the DAG member into maintenance mode. This involves moving all active databases off the server and blocking active databases from moving to the server.

https://technet.microsoft.com/en-us/library/74be3f97-ec0f-4d2a-b5d8-7770cc489919(v=exchg.150)#Pm

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