I have separate physical drives. When I changed out my Linux distro on the Linux drive Windows took back over. I don’t know what the hell happened in the bios, but things were not the same in there after that and I spent a good hour fucking with it to try and get the drive with grub on it to be the default. I finally got it working right, but it was not normal. I could manually select it at boot time no problem, but who wants to enter the boot selection on every reboot… gross.