When you add a copy of a mailbox database, error:
A server-side seed operation has failed. Error: An error occurred while performing the seed operation, which may indicate a problem with the source disk.
Database ‘_DBID_’ was not active on source server ‘_source-server-name_’.
A copy of the GUI interface does not work, but PowerShell command is:
Add-MailboxDatabaseCopy -Identity _DB name_ -MailboxServer _targetservername_ -ActivationPreference 2