Edit /etc/grub.d/40_custom with something like
Where hd1 is your Windows 8 disk and gpt2 is the partition the EFI stuff is in on that disk.
Run update-grub and reboot.
Code:
menuentry "Windows 8" { set root='(hd1,gpt2)' chainloader /EFI/microsoft/BOOT/bootmgfw.efi }
Run update-grub and reboot.
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