Hi Tamara,
I usually leave the carrots in the ground until the ground freezes. They seem to get sweeter the longer I leave them. Then I dig and cut the top but leave a stub of green. Wash them and store in plastic bags refrigerated. I have had no luck in the sand as I read it also.
As for
potatoes...I leave them in the garden cart or spread out in the sun so some of the dirt can dry off them after I dig. Don't wash them. Then I dump them in a bin in my storage room which keeps pretty cool.
I am in a moderate climate here in Montana and it may be different in Minnesota as far as when the ground freezes however.
Hope I have helped some.
LadyBug9071