If you have the following error:

——————————————————– Microsoft Exchange Error ——————————————————– Cannot activate database copy ‘Activate Database Copy…’. Activate Database Copy… Failed Error: 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 ‘db1′ on server ‘dag1′ has content index catalog files in the following state: ‘Failed’.. [Database: db1, Server: dag1] An Active Manager operation failed. Error An error occurred while trying to validate the specified database copy for possible activation. Error: Database copy ‘db1′ on server ‘dag1′ has content index catalog files in the following state: ‘Failed’..

First view / verify the copy status.

Get-MailboxDatabaseCopyStatus | fl name, contentindexstate

Update the catalog

Update-MailboxDatabaseCopy "db1/dag1" -CatalogOnly