Search for files in external storage in Finder (and if possible, Spotlight as well)
You can add the Volume to your Spotlight index.

Go to System Preferences then to Spotlight.


Click on the Privacy tab Click the + button and browse to your
external drive to add it
Now, remove the drive you just added by selecting it and clicking
the minus "-" button. (Yes, I know, crazy right! But this tells
Spotlight that the Volume exists.)
Close Spotlight Preferences.


Source: Apple Support - Spotlight: How to re-index folders or volumes.