Hay que añadir al fichero /etc/grub.d/40_custom esto:
#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
menuentry "Disco Arch Linux WDBlue1Tb" {
insmod chain
set root=(hd1,msdos1)
chainloader /EFI/WDBlue1Tb/grubx64.efi
}
menuentry "Fedora" {
insmod chain
set root=(hd3,gpt2)
chainloader /EFI/BOOT/BOOTX64.EFI
}
menuentry "Debian 11 Scholelinux" {
insmod chain
set root=(hd4,gpt1)
chainloader /EFI/debian/shimx64.efi
}
Aunque es mejor en set root asignar el uuid por si se quita/añade un disco duro.
PD: Basado en Manually add menu entry to GRUB2 menu on Arch Linux