Get Hard disk serial Number

I have used the following method in a project and it's working successfully.

private string identifier(string wmiClass, string wmiProperty)
//Return a hardware identifier
{
    string result = "";
    System.Management.ManagementClass mc = new System.Management.ManagementClass(wmiClass);
    System.Management.ManagementObjectCollection moc = mc.GetInstances();
    foreach (System.Management.ManagementObject mo in moc)
    {
        //Only get the first one
        if (result == "")
        {
            try
            {
                result = mo[wmiProperty].ToString();
                break;
            }
            catch
            {
            }
        }
    }
    return result;
}

you can call the above method as mentioned below,

string modelNo = identifier("Win32_DiskDrive", "Model");
string manufatureID = identifier("Win32_DiskDrive", "Manufacturer");
string signature = identifier("Win32_DiskDrive", "Signature");
string totalHeads = identifier("Win32_DiskDrive", "TotalHeads");

If you need a unique identifier, use a combination of these IDs.

Hm, looking at your first set of code, I think you have retrieved (maybe?) the hard drive model. The serial # comes from Win32_PhysicalMedia.

Get Hard Drive model

    ManagementObjectSearcher searcher = new
    ManagementObjectSearcher("SELECT * FROM Win32_DiskDrive");

   foreach(ManagementObject wmi_HD in searcher.Get())
   {
    HardDrive hd = new HardDrive();
    hd.Model = wmi_HD["Model"].ToString();
    hd.Type  = wmi_HD["InterfaceType"].ToString(); 
    hdCollection.Add(hd);
   }

Get the Serial Number

 searcher = new
    ManagementObjectSearcher("SELECT * FROM Win32_PhysicalMedia");

   int i = 0;
   foreach(ManagementObject wmi_HD in searcher.Get())
   {
    // get the hard drive from collection
    // using index
    HardDrive hd = (HardDrive)hdCollection[i];

    // get the hardware serial no.
    if (wmi_HD["SerialNumber"] == null)
     hd.SerialNo = "None";
    else
     hd.SerialNo = wmi_HD["SerialNumber"].ToString();

    ++i;
   }

Source

Hope this helps :)