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The best way to defrost crab is to let it sit, covered, inside the refrigerator overnight. Crab legs can also be thawed by running them under cold water for a few minutes. Even if you do not need to run the crab legs under cold water to thaw them, doing so could also help remove any remaining ice crystals or slime. Pat dry with clean paper towels before using. Arrange the crab legs in even layers inside the slow cooker and add enough water to just barely cover them.  You will need to create multiple layers of crab legs, but these layers should at least be even. Due to the shape of the crab legs, an oval slow cooker usually works better than a round one. You only need enough water to cover the crabs. Too little or too much could be result in crab legs that are too dry or not sufficiently heated. Melt the butter in a small bowl and stir in the garlic powder and dill.  If you prefer a stronger garlic taste, you can use 4 cloves of minced garlic instead of concentrated garlic powder. Since the crab legs are being cooked slowly, the flavors of the seasonings have a greater opportunity to permeate the thick shell of the legs and flavor the meat beneath. Pour the melted butter mixture over the crab legs in the slow cooker. Try to spread the butter over as many of the crab legs as possible. You can toss them to coat the crab more thoroughly, but this is not necessary. Cover the slow cooker and cook the crab legs until they are steaming hot and nearly melting inside their shells. It you did not have time to thaw your crab legs and are cooking them straight from a frozen state, add another 30 minutes to the cooking time. Use tongs to remove the crab legs from the slow cooker. Transfer them to a serving platter and enjoy while hot. If desired, you can serve the crab legs with additional melted butter or lemon wedges.

Summary:
Thaw and rinse the crab. Transfer the crab legs to a slow cooker. Mix the butter, dill, and garlic. Add the flavored butter to the crab legs. Cook on high for 4 hours. Serve hot.