As I installed Rancher OS in Hyper-V I ran into a couple of issues. Here's what they were and how to get around them.
Issue 1: The cd image doesn't boot with Hyper-V saying it didn't find a bootable device using UEFI. Solution, create your VM using Generation 1 hardware. The image doesn't support Generation 2.
Issue 2: The cd image doesn't boot with an error saying
Error process tar file(exit status 1): write /engine/docker: no space left on device. Solution, use at least 4 gigs of ram.
Hope it helps somebody.