Why am I getting an error "No Index" when I try to CREATE a spotlight index with mdutil?
Indexing can be enabled or disabled only on the root of the drive, so, "/Volumes/volName". You can read more about the command by typing man mdutil into the terminal. An excerpt:


  -i on | off:
  Sets the indexing status for the provided volumes to on or off.  Note that indexing may be delayed due to low disk space or other conditions.


Also, indexing won't work on a drive mounted with SMB. You will have to use AFP in order to do so.