Monitor only HDDs for SMART failures

modules/monitoring.nix

@@ -4,6 +4,8 @@
   # Notify when a disk starts going haywire
   services.smartd = {
     enable = true;
+    autodetect = false;
+    devices = [ { device = "/dev/sda"; } { device = "/dev/sdb"; } ];
     notifications.mail = {
       enable = true;
       recipient = "akasroua@gmail.com";