Add windows boot partition to grub. By default grub doesn't detect other operating systems. You need to get os-prober, it is an optional dependency of grub. Also by default it is disabled, so you need add a line to grub config.
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# Get optional grub dependency os-prober.
yay -S os-prober
# Enable os prober for grub.
sudo echo GRUB_DISABLE_OS_PROBER=false >> /etc/default/grub
# Regenerate grub config.
sudo grub-mkconfig -o /boot/grub/grub.cfgProgramming Language: Bash