Place potatoes in bottom of a 3-1/2-quart(or larger)slow cooker. In a large bowl combine beef,sausage,onion,3/4 cup ketchup,cracker crumbs,eggs,and salt. Mix well. Place over potatoes and shape to form a loaf.
In the same bowl,combine brown sugar,mustard and the remaining ketchup. Mix well. Spread over top of loaf,cover,and cook on the low setting 6 to 10 hours. Drain off excess liquid and serve.