SD is becoming a common feature on many servers. I understand that Microsoft, via an OEM, will support booting Hyper-V Server from USB. SD is a low-cost option for deploying servers that would get all their storage from SAN or SMB, but Microsoft needs to support (not OEMs) Windows Server booting from SD.
Stuart Daniels commented
Agree here as well for Nano and Hyper-V this is needed, its a little behind the times that this is something that can not be done already.
Tim Cerling commented
Microsoft will support Windows Server/Hyper-V booting from SD or USB for installation media but not running systems. They will support Hyper-V booting from USB if the OEM provides modified USB drivers (https://technet.microsoft.com/en-us/library/jj733589.aspx). Basically the modified driver changes the USB from removable media to fixed media so the installation process will recognize the drive as a drive it can use for installation. Since both SD and USB are seen by the operating system as removable media, installation will not recognize the device. This is a change that Microsoft needs to implement instead of the OEM making a special driver for it.
SD or Thumb Drive booting should be possible for either Hyper-V or Nano Server boot volumes.