This tool allows you to search any volume (including a network volume) regardless of the fact if it was indexed by Spotlight or not. Low Level Search might be especially useful when working with multiple external volumes and/or network volumes. The tool has an ability to save searches, which can be then put as Smart Folders into the Sidebar section “Search for”.
To open the Low Level Search go to menu Window > Find Window. At the top of the window’s toolbar choose “Search” as shown below: