Line medium-size colander with cheesecloth or 2 layers of paper toweling; place colander over a bowl. Add yogurt to colander; drain for 2 hours or refrigerate overnight. You should have about 2 cups solid yogurt and 2/3 cup liquid; discard liquid.
Place potatoes in large pot. Add enough cold water to cover. Add 2 teaspoons salt. Boil about 12 minutes or until knife-tender. Drain; let cool.
Whisk drained yogurt, sour cream, scallions, chives, lemon juice, horseradish, sugar, pepper and remaining 1 teaspoon salt in large bowl. Quarter potatoes; add to dressing. Stir gently until all pieces are coated. Refrigerate until serving. Makes 12 servings.