Okay. Seems I am getting closer to the solution.
Many people seem to have the same problem, but there is no "safe" solution.
It has been told to open the terminal use the command:
sudo dmraid -r -R /dev/sda
this should make your HD visible for installing. people who did this, have now the problem, that they have no bootloader anymore and are not able to start windows.
so this solution seems not the best for mint4win.
honestly linux mint especially mint4win seems fantastic. but userfriendly is certanly something different...