Dehydrating things with a high fat content isn't recommended. Most milk is dehydrated non fat. Guess you could pull the cream out of it (make some butter) and put the non fat portion in a pie pan and put it in the dehydrator. Would work if you have a shelf type dehydrator, like an excalibur. Problem is, you won't get many flakes from it, and you'll use lots in electricity. Not really worth it.
Raw milk: I'd freeze it, or pressure can it, or make butter from the cream, or make cheese.
I have not freeze dried milk. Not sure how that would work. We have a freeze dryer arriving in July.