diff --git a/packages/checks/modules/nixos/external-storage.nix b/packages/checks/modules/nixos/external-storage.nix
new file mode 100644
index 0000000..d89f9df
--- /dev/null
+++ b/packages/checks/modules/nixos/external-storage.nix
@@ -0,0 +1,12 @@
+{ config, lib, ... }:
+
+{
+  systemd.tmpfiles.settings."00-testing-external-storage-underlays" = lib.mapAttrs' (name: cfg: {
+    name = cfg.mountpoint;
+    value.d = {
+      user = toString cfg.uid;
+      group = toString cfg.gid;
+      mode = "0700";
+    };
+  }) config.services.external-storage.underlays;
+}