Turned out that while UEFI was enabled in BIOS windows 10 was installed in legacy mode. This happens if your OS was installed before UEFI was enabled or sometimes if you had BIOS set to compatibility mode i.e BIOS first UEFI second boot orders etc when the original install (in my case this was because my original install was actually windows 7 on a different mobo! yes i have done ~100 build to build upgrades since then, anywho, back to the plot).
This used to require a complete re-install but can now easily be corrected thanks to a new MS tool called MBR2GPT available in the creators update.
- to check if your win10 is in legacy or UEFI mode run msinfo32 and look at the bios mode section in system summary
if it says UEFI this won't fix for you, if it says legacy these steps will work. (note you still need to meet the other requirements about HDCP etc)
- You are going to need covert your install from an MBR to GPT install
- Luckily there is an official way to do this in windows 10 creator update and beyond. https://docs.microsoft.com/en-us/window ... mbr-to-gpt
- Requires latest winpe - get from 1703 version of ADK (do not use any other version, only ever use official MS winpe, not ones you find on interwebs) https://developer.microsoft.com/en-us/w ... oyment-kit
- use these winpe creation instructions: https://docs.microsoft.com/en-us/window ... able-drive
- if you hit error -2147212243 with the makewinpemedia command use this fix https://www.virtualizationhowto.com/201 ... 147212243/
remember to change the drive letter in the command from f: to p: if you followed exactly the steps in step 5 - also pay attention to using the x86 or amd64 - i reccomend using the latter - so change the line in this manual copy as needed
Good luck and remember backup before you do this, this is messing with your disks.