Have you thought about adding the user the ability to add a new filter to the filters that will be assigned to a given directory?
To quickly search for files only from a given directory / disk.
You could then create several ready-made filters and change them efficiently.
I know that there is an option to enter the folder path in quotation marks in the search field, but this option is quite burdensome for my friends unfamiliar with computers.
After typing a search query in the search bar, you can save it as a filter by choosing Search from the menu bar, followed by Add to Filters....
You can change your filters through Menu:Search > Organize Filters
Menu:View > Filters wil show a dropdownlist of all your filters to choose from, next to the search bar.
An alternative would be to save your search as a bookmark (Menu:Bookmarks > Add to Bookmarks..
