Hello,
I could solve it by a detour. It seems Windows 10 does not bring natively disk drivers for my old motherboard but Windows 7 Installation has not even asked for it, since I had not USB device connected.
After installing Windows 7, I installed every driver for Windows 7 I could get from asus to make sure Windows 10 Installer is not missing anything. From within Windows 7 I could start installation of Windows 10.
(By the way, I dared to pull the sata-cable to my linux device because I have not wanted Windows to destroy my grub bootloader. Luckily it was very easily like this and no other difficulties.)
Patrick, I agree with you, that the Windows 10 Installer has indeed needed a media driver / disk controller. But why does it not bring the basic drivers as Linux distributions are doing it.
Thank you for your help! The hint with the disk controller was helpful and made me try earlier the final solution.