Create Smart Folder which only shows files of Macintosh HD
When creating your Smart Folder through File → New Smart Folder, choose your Macintosh HD before you create it (either from the sidebar or through general navigation).

Then, in your Smart Folder criteria, you can choose to search only Macintosh HD.



My Macintosh HD is called Mac SSD in the example above.

When you click Save, the Smart Folder that is created maintains this setting.