Include files from secondary hard drive in spotlight searches
All hard drives (both internal and external) can be searched though Spotlight. It's possible that it may need to be re-indexed.

To do this, go to System Preferences > Spotlight, then click on "Privacy". Click on the "+" button and select the hard drive that you wish to re-index. After that, select the hard drive within the Privacy list, and select the "-" button.

It should start to re-index the hard drive in question (you should see a progress bar in Spotlight when you click on it).