In a large bowl, combine ground beef, onion, eggs, cracker crumbs, 1/4 cup ketchup, milk, Worcestershire sauce, salt, pepper and garlic powder.
Line the slow cooker crock with nonstick aluminum foil.
Shape the meat mixture into a loaf shape and place in foil-lined slow cooker crock.
In a small bowl, combine remaining ketchup, brown sugar and mustard. Spread over top of meatloaf.
Cover slow cooker and cook on HIGH for 2 1/2 to 3 hours or LOW for 4 to 5 hours. If using a slow cooker with a probe, cook until meatloaf reaches 160°F.