The new search in nautilus searches recursively starting at the directory where the search is initiated
So if you open at your home folder it will search there & in any dir/sub dir.'s inside your home folder.
If you wanted to search an external partition then open that partition in nautilus & search from there.
If you wanted to search the complete filesystem then you'd go to / in nautilus & search from there (Computer
Obviously the farther away the longer a search may take so makes sense to shorten the search paths
If you want, the old "Search for Files' app can be installed & used - screen
Myself find the nautilus search to work quite well & rarely use gnome-search-tool anymore
sudo apt-get install gnome-search-tool