In your Exchange 2010 environment, you may be performing DAG fail-overs to perform maintenance etc. but sometimes it doesn’t always work as you want it to.

When you initiate the fail over either from PowerShell or the Exchange Control Panel, you might encounter the following error Below is the command used:

Move-ActiveMailboxDatabase -Server MBX1 -ActivateOnServer MBX2 -Confirm:$All

The error is a bit deceiving, especially when you have not performed a block on the database copy. To fix this error is pretty simple actually.

All you need to do is suspend the database copy in question and then resume it. Once it has synchronized and is back in a Healthy State, try and run the command as shown above and it should succeed.

Hope it helps.