FindFirstDevice wince error 18

Question

I have a c# application running on winCE 6.0 version.  I need to unload/reload the SD card driver at run time.  I am attempting to do this via calling FindFirstDevice, then DeactivateDevice/ActivateDeviceEX calls.  My problem is the FindFirstDevice() call always fails.  I believe it to be a problem with the way I am marshalling the second parameter to it.  Can anyone tell me what I am doing wrong?  Here is the code:

      [DllImport("coredll.dll", SetLastError = true)]

      public static extern int FindFirstDevice(DeviceSearchType

      searchType, IntPtr searchParam, ref DEVMGR_DEVICE_INFORMATION pdi);

      public bool MountSDCardDrive(string mRegPath)

      {

         const int INVALID_HANDLE_VALUE = -1;

         

         int handle = INVALID_HANDLE_VALUE;

         DeviceSearchType searchType = DeviceSearchType.DeviceSearchByDeviceName;

         DEVMGR_DEVICE_INFORMATION di = new DEVMGR_DEVICE_INFORMATION();

         di.dwSize = (uint)Marshal.SizeOf(typeof(DEVMGR_DEVICE_INFORMATION));

         string searchParamString = "*";

         IntPtr searchParam = Marshal.AllocHGlobal(searchParamString.Length);

         Marshal.StructureToPtr(searchParamString, searchParam, false);

         handle = FindFirstDevice(searchType, searchParam, ref di);

         if (handle == INVALID_HANDLE_VALUE)

         {

            // Failure - print error

            int hFindFirstDeviceError = Marshal.GetLastWin32Error();

            using (StreamWriter bw = new StreamWriter(File.Open(App.chipDebugFile, FileMode.Append)))

            {

               String iua = "DevDriverInterface: error from FindFirstDevice: " + hFindFirstDeviceError.ToString();

               bw.WriteLine(iua);

            }

            return false;

         }

... (rest of code)

If I change the line "Marshal.StructureToPtr(searchParamString, searchParam, false);" to "searchParam = Marshal.StringToBSTR(searchParamString);" I end up with error 1168 (ERROR_NOT_FOUND) instead of 18 (no more files).  

Note my intent is to use a searchParamString of "SDH1" when I get this working.  I am currently using searchParamString of "*" as examples I see on the web indicate that should always return something.  

Thanks for any help you can give  - Lynn