I agree with Patio.
Here is what I have done. I gave upon doing Dual-boot with Windows and GRUB. It can be done, but it is just too hard.
Instead, I put versions of Linux on small laptop drives that can get power from the USB port. Linux has not issue with booting from a USB device. I disable my main drive before doing a Linux install on a laptop drive using USB. GRUB never sees my main drive, so it leaves it alone. (This is only done once.)
After that, I let my BIOS see the main drive again. From that time on, if I want a Linux OS, I select USB device in the BIOS and it boots up Linux. No problem. No errors.
The advantage ist hat you do no ha ve to study GRUB to make it work. My Windows boot loader supports Windows XP,Windows 7 and Windows 10. Whenever I want Linux, I just have to hit F12 early in the POST and a BIOS menu comes up and asks meto selet a boot device. I can select the HDD, the opticaldrive or whaever USB device. Somehow the BIOS knows if the USB device is bootable.
SoI have completely forgotton whateverI knew about GRUB and do not regret it.