Can I make sourdough starter with raw potatoes?
Yes!
I'm currently attending Le Cordon Bleu for Baking and Patisserie.  I'm in the breads class right now, and my text book has a recipe for a potato sour with raw potato.  Here..

8 oz Bread Flour
6.5 oz Warm Water
.16 oz (1 tsp) Salt
.16 oz (1 tsp) Sugar
1 Large Potato, peeled


Mix together the flour, water, salt, and sugar into a smooth soft dough.  Add potato.
Place in sterilized bowl. Cover tightly with muslin or other clean fabric so the starter can breathe.  Let rest in a warm place for up to 24 hours, until the mixture becomes frothy.
Stir well and cover with plastic film.  Leave to stand 2-3 days in a warm place, until the mixture becomes light and foamy.  Stir thoroughly each day.
Pour the fermented starter to a glass jar and store in the refrigerator for approximately 3 days, or until a clear liquid collects on top of the mixture.  This indicates the mixture is ripened enough for use.Carefully pour all the liquid collected on the surface into a measuring jug, discarding the solid mixture that remains on the bottom.  The weight of the liquid should be greater than the weight of the water used, because some of the flour will be poured off with the liquid.