nixpkgs/nixos/modules/hardware/usb-storage.nix

{ config, lib, pkgs, ... }:
{
  options.hardware.usbStorage.manageStartStop = lib.mkOption {
    type = lib.types.bool;
    default = true;
    description = ''
      Enable this option to gracefully spin-down external storage during shutdown.
      If you suspect improper head parking after poweroff, install `smartmontools` and check
      for the `Power-Off_Retract_Count` field for an increment.
    '';
  };

  config = lib.mkIf config.hardware.usbStorage.manageStartStop {
    services.udev.extraRules = ''
      ACTION=="add|change", SUBSYSTEM=="scsi_disk", DRIVERS=="usb-storage", ATTR{manage_system_start_stop}="1"
    '';
  };
}