Never, ever ever let Microsoft manage your partitions. I made a fatal mistake of clicking "upgrade volume to dynamic". Not knowing what the true horrible implications of this was, I foolishly clicked "yes". No warnings that this was an irreversible process. Then suddenly had to reboot. The machine blue-screened first time, until I went into partition manager to fix it. It now boots fine.

However the disk is now borked in a proprietary format that I can't do anything with except wipe it and start again. I can't find the "dmunroot" utility which wipes all other partitions anyway.

Luckily it is my spare machine, but it will take ages to set up again.

Microsoft's attitude is "why would anybody wish to downgrade?"