How to search multiple spotlight sources / drives simultaneously?
You can use HoudahSpot to search multiple locations.

HoudahSpot is an advanced front-end to Spotlight. It lets you build complex queries to search metadata stored in the Spotlight index.

In the HoudahSpot "Query Pane" you will find a "Where…" section. Here you can specify which locations / drives / folders to search. There is also an "Exclude…" section where you can specify folders to ignore. E.g. you could search your Home folder, but ignore results from the ~/Library folder.

Full disclosure: I am the developer of HoudahSpot