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Spray 6-quart slow cooker with non-stick spray.
Rinse the uncooked pasta well in cold water and drain.
Add uncooked pasta to crockpot along with milks, cheeses, salt, pepper, mustard, garlic and cayenne pepper. Stir to combine, making sure macaroni is submerged in liquid as much as possible.
Dot with cubed butter.
Cover and cook on low heat for 1 hour. Remove lid and stir. Depending on your slow cooker, your dish could be done at this point or require UP TO 1-2 more hours.
If not done, continue cooking and checking periodically (ever half hour or so).
When done, pasta will be tender and liquid will be thick and creamy. Sauce will thicken even more after lid is removed and mac and cheese sits.