Fast Startup (Fast Boot) feature of MS Windows 10 is really useful, in a modern PC it allows to boot up in few seconds, but in some circumstances it’s better to disable it in order to install a second operating system.
So, in order to disable it, you just have to follow this guide:
Press the Windows Key + R to bring up the Run dialog, type powercfg.cpl and press Enter
Click on “Choose what the power buttons do” in the left menu panel;
Disable “Turn on fast startup (recommended)” and press “Save changes”
I hope this guide could be useful.