Share external FAT formatted drive over SMB in Mountain Lion
You don't have to manually edit a configuration file; it's possible to share drives via SMB using the Sharing control panel. Under the File Sharing service, hit the "Options..." button to enable SMB:



This will make your shared folders accessible via SMB to the specified users. It shouldn't matter what the source filesystem is (FAT, HFS, or other) as long as your Mac can access it.

If you don't see any contents from the client, make sure you've added your drive as a shared folder, and that you've given users the appropriate permissions.