When working in HA environments it is possible for one of the mailbox database copies to be Failed and Suspended.
In this situation depending on what is wrong you can attempt to resume the copy or update the copy. The situation I came across recently was giving the following error:
A source-side operation failed. Error An error occurred while performing the seed operation. Error: The process cannot access the file ‘CiPT0000.000’ because it is being used by another process..
I was able to find information in the following article to help resolve the issue. https://social.technet.microsoft.com/Forums/exchange/en-US/19387e3b-95e7-47da-bea1-682947b71cfd/dag-copy-resedding-error?forum=exchange2010
I was required to:
· Stop the Search Service
· Deleted the Catalog Directory
· Start the Search Service
· $Exscripts: .\ResetSearchIndex.ps1 -force "DB_Name"
· Update-MailboxDatabaseCopy -CatalogOnly