If you see the error in the event log with an ID of 123 and source ExchangeStoreDB :-
At ‘3/16/2012 11:42:51 AM’ the Microsoft Exchange Information Store Database ‘DATABASENAME’ copy on this server experienced a corrupted search catalog. Consult the event log on the server for other “ExchangeStoreDb” and “MSExchange Search Indexer” events for more specific information about the failure. Reseeding the catalog is recommended via the ‘Update-MailboxDatabaseCopy’ task.
Then you need to reseed the catalogs on the backup server.
Carry out the following steps.
1) Suspend the Database on the backup server. Right click, suspend.
2) Open Exchange Management Shell, Run the command.
Update-MailboxDatabaseCopy –Identity DATABASENAME\BACKUPSERVER –SourceServer MOUNTEDSERVER
3) This will then copy across the catalogs
4) Unsuspend it
Source : Technet