The Problem:

Trying to install Windows Server 2016 R2 Standard on Hyper-V, I run into this error:

Load driver
A media driver your computer needs is missing.  This could be a DVD, USB or Hard disk driver.  If you have a CD, DVD, or USB flash drive with the driver on it, please insert it now. 

Note: If the installation media for Windows is in the DVD drive or on a USB drive, you can safely remove it from this step.

The Fix:
Re-download windows ISO from MSDN. Worked fine after that.