Get-MailboxDatabaseCopyStatus -server "SERVERNAME"
From the output of the above command you will notice the ContentIndexState shows "Failed" for the database copy. The end result here is that you can't mount the database unless you update the search catalog from another healthy copy in the DAG. If you try to activate the copy you will see an error such as:
"An Active Manager operation failed. Error: The database action failed. Error: An error occurred while trying to validate the specified database copy for possible activation. Error: Database copy 'DB01' on server 'Server01.contoso.com' has content index catalog files in the following state: 'Failed'.
To resolve this, run the following PowerShell command and be sure to specify the database name and servername hosting the failed copy:
To confirm the catalog index has been fixed, re-run the Get-MailboxDatabaseCopyStatus command again. Notice the state is now "Healthy". Try the Move-ActiveMailboxDatabase command again and it should work.
What if I only have one copy and I need to activate it?
If this is the case, you can issue the following command to move the database copy and make it active without validating the content index:
Move-ActiveMailboxDatabase "DATABASE" -ActivateOnServer "SERVERNAME" -SkipClientExperienceChecks
I encountered the above situation when performing a DR datacenter switchover at a customer site and this resolved not being able to mount the database.