Hey boredog, do you remember what registry key they had you add to get it working again? I'm in a very similar or possibly the same situation after doing a clean reinstall of the latest Windows insider build. :-(
I had this same problem and this reg key fixed it but I have a new problem after latest insider update. If I go into shadow mode do my thing and reboot out of shadow mode, my pc becomes unbootable. I then have to use the fix boot issues in Marcrium to get my boot back. Contacted the developer and he never did get back to me. That was at least two weeks ago. I had paid for the software but had to uninstall it because he doesn't appear to be going to fix it. Most insider builds evenyualy go to everyone as far as I know.
Here is the reg fix but just be ware the latest insider build will make your pc unbootable.
start a command prompt as an administrator then type the following command and press Enter:
reg add HKLM\SYSTEM\CurrentControlSet\Services\diskpt /v Control /t REG_DWORD /d 268435456 /f
Reboot and try again.