Brown the ground beef in a skillet over medium heat until fully cooked. Break it into crumbles and drain the fat.
Stir together the spaghetti sauce and 1 1/2 cups water. Lightly grease a 6-quart slow cooker and spread a thin layer of sauce in the bottom.
Layer half the broken spaghetti, half the beef, and half the remaining sauce. Press the noodles below the liquid. Add 1 cup mozzarella. Repeat with the remaining spaghetti, beef, and sauce, reserving the last cup of cheese.
Cover and cook on LOW for 2 to 3 hours, checking at 2 hours. The pasta should be tender and the center hot. Add a small splash of hot water if any strands remain dry.
Sprinkle with the remaining mozzarella, cover for 10 to 15 minutes until melted, then turn off the heat and rest for 10 minutes before serving.