Clever GRUB, I hear you say.
I don’t often boot into Windows, so I can’t say for sure when this happened but I would guess that it was when I upgraded to Cinna back in February. Either way, though, it’s surprisingly easy to fix once you have figured out where to look.
There are plenty of guides on t’interwebs that guide you through re-installing GRUB, but since it was just the menu I wanted to fix I went straight to the final step which involves rebuilding the grub.cfg file.
This file can be found in /boot/grub so…
First, take a copy of your current file just in case things go completely pear-shaped:
# cd /boot/grub
# mv grub.cfg grub.cfg.sav
Then rebuild your grub.cfg:
# grub-mkconfig -o grub.cfg
And that’s it.
And now that I have successfully booted into both partitions, I have deleted the grub.cfg.sav backup file.