Get serial number only once

Pilz.Cryptography/UniquieID.cs

@@ -72,7 +72,10 @@ namespace Pilz.Cryptography
             return hash;
         }
 
+        private static string Win32_PhysicalMedia_SerialNumber = null;
         private static string TryGetSerialNumberOfFirstHardDrive()
+        {
+            if (Win32_PhysicalMedia_SerialNumber == null)
             {
                 var searcher = new ManagementObjectSearcher("SELECT * FROM Win32_PhysicalMedia");
                 var sn = string.Empty;
@@ -83,7 +86,10 @@ namespace Pilz.Cryptography
                         sn = wmi_HD["SerialNumber"].ToString().Trim();
                 }
 
-            return sn;
+                Win32_PhysicalMedia_SerialNumber = sn;
+            }
+            
+            return Win32_PhysicalMedia_SerialNumber;
         }
 
         public override string ToString() => ID;