In: https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/system/boot/loader/grub/grub.nix#L116

         description = ''
           The device on which the GRUB boot loader will be installed.
           The special value <literal>nodev</literal> means that a GRUB
           boot menu will be generated, but GRUB itself will not
           actually be installed.  To install GRUB on multiple devices,
           use <literal>boot.loader.grub.devices</literal>.
+          Disk serial id/wwn ("/dev/by-id/XXXX") are also accepted.
        '';

Disk serial id's and world wide names can be looked up with "ls -l /dev/disk/by-id/"
Explanation: https://superuser.com/a/874168