in the comments as part of the link you will discover this
To make sure the external volume in question is indeed set to index its contents, enter
mdutil -s /Volumes/Volume_Name
and hit Return.
To turn indexing on if the previous command reports that it's disabled, enter
sudo mdutil -i on /Volumes/Volume_Name
and hit Return. Terminal will request an administrator password; enter it (nothing will be displayed) and hit Return.
If all goes well, the Terminal response will be
/Volumes/Volume_Name:
Indexing enabled.