Hi Jonas,
After booting of Windows, its boot drive must be C.
I'd like to confirm or to know:
1) do you have a dual- (or multi-) boot machine, or install W10 Insider Preview on a virtual machine?
2) please provide a view of Disk Management
Regards,
Ashidacchi -- https://ssl01.rocketnet.jp/hokusosha.com/default.html