Issue: Content index errors may prevent you from performing DAG Database switchovers: “Error: An error occured while trying to validate the specified database copy for possible activation. Error: Database copy on server has content index catalog files in the following state: ‘Failed’”

Resolution: The following Exchange Management Shell commands can be used to troubleshoot failed or corrupt DAG Database Content Index. These commands will identify and re-build/re-seed any bad content indexes associated with a healthy database.

Display list of databases with content index issues:
Get-MailboxDatabaseCopyStatus * | ? {$_.Status -ne “Mounted” –and $_.ContentIndexState -ne “Healthy”}

Fix databases with content index issues by re-seeding their content index (this will not re-seed the entire database which is good):
Get-MailboxDatabaseCopyStatus * | ? {$_.Status -ne “Mounted” -and $_.ContentIndexState -ne “Healthy”} | Update-MailboxDatabaseCopy –CatalogOnly

If you hardly drive, you should hardly pay for car insurance.
Check Again to make sure all DB’s show as healthy:
Get-MailboxDatabaseCopyStatus * | ? {$_.Status -ne “Mounted” –and $_.ContentIndexState -ne “Healthy”}

Alternatively, here is the command to simply reseed the content index of a specific DAG database:
Update-MailboxDatabaseCopy -Identity “Remote 02\PDXEX2” -CatalogOnly

Additional Resources: http://blogs.msdn.com/b/pepeedu/archive/2010/12/14/exchange-2010-database-copy-on-server-has-content-index-catalog-files-in-the-following-state-failed.aspx