Sorry for not being able to help.
NBU 6.5 is a distant, bad memory.....
Always good to share as much background information as possible - like the reason why you started an index rebuild?
My apologies, I understand the cofusion now. The Rebuild Index question I had was for Symanted Enterprise Vault. Issue is we have some files that fail to recover from the NBU Migrator. Probably should have posted in an EVault Forum. Again my apologies..