Mount output:
mount
devtmpfs on /dev type devtmpfs (rw,nosuid,size=388432k,nr_inodes=969945,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=3,mode=620,ptmxmode=000)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev,size=3884288k)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
tmpfs on /run type tmpfs (rw,nodev,size=1942144k,mode=755)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on / type tmpfs (rw,relatime,mode=755)
/dev/sdb1 on /iso type iso9660 (ro,relatime)
/dev/loop0 on /nix/.ro-store type squashfs (ro,relatime)
tmpfs on /nix/.rw-store type tmpfs (rw,relatime,mode=755)
unionfs on /nix/store type fuse.unionfs (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other)
unionfs on /nix/store type fuse.unionfs (ro,relatime,user_id=0,group_id=0,default_permissions,allow_other)
ramfs on /run/keys type ramfs (rw,relatime)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/run/current-system/systemd/lib/systemd/systemd-cgroups-agent,name=systemd)
cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
cgroup on /sys/fs/cgroup/net_cls type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=29,pgrp=1,timeout=0,minproto=5,maxproto=5,direct)
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
mqueue on /dev/mqueue type mqueue (rw,relatime)
fusectl on /sys/fs/fuse/connections type fusectl (rw,relatime)
tmpfs on /run/user/0 type tmpfs (rw,nosuid,nodev,relatime,size=776856k,mode=700)
/dev/sda3 on /mnt type btrfs (rw,relatime,space_cache,subvolid=5,subvol=/)
/dev/sda1 on /mnt/boot type ext2 (rw,relatime,errors=continue,user_xattr,acl)

configuration.nix:
# Edit this configuration file to define what should be installed on
# your system.  Help is available in the configuration.nix(5) man page
# and in the NixOS manual (accessible by running ‘nixos-help’).

{ config, pkgs, ... }:

{
  imports =
    [ # Include the results of the hardware scan.
      ./hardware-configuration.nix
    ];

  # Use the GRUB 2 boot loader.
  boot.loader.grub.enable = true;
  boot.loader.grub.version = 2;
  # boot.loader.grub.efiSupport = true;
  # boot.loader.grub.efiInstallAsRemovable = true;
  # boot.loader.efi.efiSysMountPoint = "/boot/efi";
  # Define on which hard drive you want to install Grub.
  boot.loader.grub.device = "/dev/sda"; # or "nodev" for efi only

  # networking.hostName = "nixos"; # Define your hostname.
  # networking.wireless.enable = true;  # Enables wireless support via wpa_supplicant.
  networking.connman.enable = true;

  # Select internationalisation properties.
  # i18n = {
  #   consoleFont = "Lat2-Terminus16";
  #   consoleKeyMap = "us";
  #   defaultLocale = "en_US.UTF-8";
  # };

  # Set your time zone.
  time.timeZone = "Pacific/Vancouver";

  # List packages installed in system profile. To search by name, run:
  # $ nix-env -qaP | grep wget
  # environment.systemPackages = with pkgs; [
  #   wget
  # ];

  # List services that you want to enable:

  # Enable the OpenSSH daemon.
  # services.openssh.enable = true;

  # Enable CUPS to print documents.
  # services.printing.enable = true;

  # Enable the X11 windowing system.
  services.xserver.enable = true;
  services.xserver.layout = "us";
  services.xserver.xkbOptions = "eurosign:e";

  # Enable the KDE Desktop Environment.
  # services.xserver.displayManager.kdm.enable = true;
  # services.xserver.desktopManager.kde4.enable = true;
  services.xserver.displayManager.sddm.enable = true;
  services.xserver.desktopManager.enlightenment.enable = true;

  # Define a user account. Don't forget to set a password with ‘passwd’.
  # users.extraUsers.guest = {
  #   isNormalUser = true;
  #   uid = 1000;
  # };

  # The NixOS release to be compatible with for stateful data such as databases.
  system.stateVersion = "16.09";

}