Mount disks on NixOS startup

Edit configuration.nix:

fileSystems = {
    "/mnt/HDD1".label = "HDD1";

    "/mnt/HDD2".device = "/dev/sdc1";

    "/mnt/HDD3" = {
        device = "/dev/hda2";
        fsType = "ext3";
        options = [ "data=journal" ];
    };
};

Add other filesystems with:

boot.supportedFilesystems = [ "ntfs" ];