Iridium:
Couldn't mount the drive manually.
BUT as Klojum in the first posting said, the drive was in hibernation mode.
Windows 10 never shuts down completely. (you have to deactivate the fast boot option).
After shutting down and switching back to my living room pc, everything works fine!
Thanks everyone, and sorry that I haven't checked the hibernation mode thing first....
One of the best support I ever had.
Greetings
Markus