Place ribs, meaty-side up, in a large roasting pan. Sprinkle ribs with salt and pepper.
Bake at 475° for 30 minutes. Meanwhile, combine ketchup and next 4 ingredients in a small bowl.
Arrange ribs in a 6-qt. slow cooker, adding sauce on each layer of ribs. Depending on the shape of your slow cooker—oval or round—you may have to cut each rib rack into thirds instead of in half. Cover and cook ribs on LOW 9 hours. Remove ribs from slow cooker; cover to keep warm.
Pour drippings and sauce from slow cooker into a saucepan. (Skim a few ice cubes across the surface of sauce to remove fat, if desired, and discard.) Bring sauce to a boil; reduce heat, and simmer over medium heat 20 minutes or until sauce thickens. (Sauce will reduce by about half.) Brush sauce over ribs before serving.
Make Ahead: These ribs can hold for several hours after finishing in the slow cooker. Place sauced-up ribs on a rimmed baking sheet; cover tightly with foil. Place in a preheated 190° oven to keep warm.