diff --git a/NixOS.md b/NixOS.md
index a0dd5f0..4567d74 100644
--- a/NixOS.md
+++ b/NixOS.md
@@ -236,6 +236,8 @@ nix-eval '{ a=false; }.a or true'
 #=> false
 nix-eval '{ a=1; }?a'
 #=> true
+nix-eval 'with lib.strings; concatMapStrings (x: " -a " + x + " -i 600") ["sda" "sdc"]'
+#=> " -a sda -i 600 -a sdc -i 600"
 ```
 
 ## Escape strings
diff --git a/hosts/yodaHedgehog/configuration.nix b/hosts/yodaHedgehog/configuration.nix
index fff6be8..a6d3768 100644
--- a/hosts/yodaHedgehog/configuration.nix
+++ b/hosts/yodaHedgehog/configuration.nix
@@ -22,7 +22,8 @@
       ../../modules/btrbk
       ./btrbk-config.nix
       ../../modules/de-p1st-monitor.nix
-      ../../modules/spin-down-hdd.nix
+      ../../modules/spin-down/hdparm.nix
+      ../../modules/spin-down/hd-idle.nix
 
       ../../modules/btrfs-scrub.nix
       ../../modules/btrfs-mount-options.nix
@@ -32,6 +33,14 @@
   boot.initrd.luks.devices."crypted".allowDiscards = true;
   yoda.btrfsFileSystems = ["/" "/mnt/backup"];
   #yoda.btrfsMounts = yoda.btrfsFileSystems;
+  yoda.spin-down.hdparm = [
+    # 6tb1
+    "ata-WDC_WD60EFAX-68SHWN0_WD-WX31D29924PZ"
+  ];
+  yoda.spin-down.hd-idle = [
+    # 6tb2; not supported by hdparm / does not support setting idle timer
+    "ata-ST6000DM003-2CY186_ZR11WA9K"
+  ];
 
   boot.kernelParams = [];
   boot.kernelPackages = pkgs.linuxPackages;
diff --git a/hosts/yodaNas/configuration.nix b/hosts/yodaNas/configuration.nix
index 9d5221c..daabc72 100644
--- a/hosts/yodaNas/configuration.nix
+++ b/hosts/yodaNas/configuration.nix
@@ -26,7 +26,8 @@
       ./btrbk-config.nix
       ../../modules/netcup-dns.nix
       ../../modules/de-p1st-monitor.nix
-      ../../modules/spin-down-hdd.nix
+      ../../modules/spin-down/hdparm.nix
+      ../../modules/spin-down/hd-idle.nix
 
       ../../modules/btrfs-scrub.nix
       ../../modules/btrfs-mount-options.nix
@@ -36,7 +37,7 @@
   boot.initrd.luks.devices."luks-3d974bd0-f373-469b-8e9c-2d5516e9f0f5".allowDiscards = true;
   yoda.btrfsFileSystems = ["/" "/mnt/data" "/mnt/backup"];
   #yoda.btrfsMounts = yoda.btrfsFileSystems;
-  yoda.spin-down-hdd = [
+  yoda.spin-down.hdparm = [
     # 4tb1
     "ata-WDC_WD40EFRX-68N32N0_WD-WCC7K0CPF0N1"
     # 3tb1
@@ -44,6 +45,7 @@
     # 3tb2
     "ata-WDC_WD30EFRX-68EUZN0_WD-WMC4N0564095"
   ];
+  yoda.spin-down.hd-idle = [];
 
   boot.kernelParams = [
     # Microarchitectural Data Sampling (MDS), see https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html#mitigation-control-on-the-kernel-command-line
diff --git a/modules/spin-down/hd-idle.nix b/modules/spin-down/hd-idle.nix
new file mode 100644
index 0000000..3d3aebd
--- /dev/null
+++ b/modules/spin-down/hd-idle.nix
@@ -0,0 +1,33 @@
+{ lib, config, options, pkgs, modulesPath, ... }:
+with lib;
+let
+  cfg = config.yoda.spin-down.hd-idle;
+in
+{
+  options = {
+    yoda.spin-down.hd-idle = mkOption {
+      type = types.listOf types.str;
+      default = [ ];
+      example = ["ata-ST6000DM003-2CY186_ZR11WA9K"];
+      description = ''
+        List with IDs (/dev/disk/by-id/<ID>) of HDDs to spin down with hd-idle after 5 minutes.
+
+        Please try to use yoda.spin-down.hdparm first.
+      '';
+    };
+  };
+
+  config = mkIf (length cfg > 0) {
+    systemd.services."hd-idle" = {
+      description = "Spin down inactive HDDs";
+      wantedBy = [ "multi-user.target" ];
+      script =
+        let
+          args = strings.concatMapStrings (x: " -a disk/by-id/${x} -i 300") cfg;
+        in ''
+          set -eu -o pipefail
+          ${pkgs.hd-idle}/bin/hd-idle ${args}
+        '';
+    };
+  };
+}
diff --git a/modules/spin-down-hdd.nix b/modules/spin-down/hdparm.nix
similarity index 85%
rename from modules/spin-down-hdd.nix
rename to modules/spin-down/hdparm.nix
index f4117f6..b82a0ff 100644
--- a/modules/spin-down-hdd.nix
+++ b/modules/spin-down/hdparm.nix
@@ -16,7 +16,7 @@
 # Get power status:
 #   sudo hdparm -C /dev/disk/by-id/ata-WDC_WD40EFRX-68N32N0_WD-WCC7K0CPF0N1
 #   #=> drive state is: active/idle
-#
+
 # Spin down after 2 minutes (120s/5s = 24).
 # TODO: Western Digital (WD) Red/Green drives don't work properly. For low `-S` values, they spin down after **10** minutes. And for high values they don't spin down at all.
 # - https://superuser.com/questions/1746074/spin-down-not-working-on-wd-red
@@ -47,26 +47,34 @@
 
 # See also: Systemd service to spin down after boot. https://wiki.archlinux.org/title/hdparm#Putting_a_drive_to_sleep_directly_after_boot
 
-# TODO: For external (USB-)disks, use `hd-idle`.
+# If a (external) disk does not supprot setting a timer with hdparm, use `hd-idle` instead.
 # https://www.reddit.com/r/NixOS/comments/751i5t/comment/do3f3l7/
 #   # Options:
-#   # -a <disk>: Select disk for subsequent parameters.
+#   # -a <disk path without /dev>: Select disk for subsequent parameters.
 #   # -i 120: Spin down after 2 minutes (120 seconds) of inactivity.
-#   ExecStart = "${pkgs.hd-idle}/bin/hd-idle -a /dev/disk/by-id/XXX-XXX-XXX -i 120";
+#   ExecStart = "${pkgs.hd-idle}/bin/hd-idle -a disk/by-id/XXX-XXX-XXX -i 120";
+
+# Example usage:
+#   sudo smartctl -i -n standby /dev/disk/by-id/ata-WDC_WD60EFAX-68SHWN0_WD-WX31D29924PZ | grep 'Power mode'
+#   #=> Power mode is:    ACTIVE or IDLE
+#   sudo hd-idle -a disk/by-id/ata-WDC_WD60EFAX-68SHWN0_WD-WX31D29924PZ -i 120
 
 { lib, config, options, pkgs, modulesPath, ... }:
 with lib;
 let
-  cfg = config.yoda.spin-down-hdd;
+  cfg = config.yoda.spin-down.hdparm;
 in
 {
   options = {
-    yoda.spin-down-hdd = mkOption {
+    yoda.spin-down.hdparm = mkOption {
       type = types.listOf types.str;
       default = [ ];
       example = ["ata-WDC_WD40EFRX-68N32N0_WD-WCC7K0CPF0N1"];
       description = ''
-        List with IDs (/dev/disk/by-id/<ID>) of HDDs to spin down after 10 minutes.
+        List with IDs (/dev/disk/by-id/<ID>) of HDDs to spin down with hdparm after 2-10 minutes.
+
+        Not all HDDs (e.g. Seagate Barracuda ST600DM003) and many external disk enclosures don't support setting the idle timer.
+        In that case, please use yoda.spin-down.hd-idle for those disks instead.
       '';
     };
   };