Cross thread exception in code- if i execute this code say 10 times 9 times it runs without any problem but 1 time it gives cross thread exception.i am unable to understand why this is happening....

    public interface IUFDHelper
    {
        IList<string> ufdDriveList { get; set; }
        ManagementScope scope { get; set; }
        WqlEventQuery query { get; set; }
        ManagementEventWatcher watcher { get; set; }
        ManagementObjectSearcher searcher { get; set; }
        IList<string> MediaConnectWatcher_EventArrived(object sender, EventArrivedEventArgs e);
    }
</string></string>

public class UFDDetect : IUFDHelper
    {
        #region Private Mambers
        private IList<string> _driveList;
        private ManagementScope _scope;
        private WqlEventQuery _query;
        private ManagementEventWatcher _watch;
        private ManagementObjectSearcher _searcher;
        #endregion
        #region interface mambers
        public IList<string> ufdDriveList
        {
            get
            {
                return _driveList;
            }
            set
            {
                _driveList = value;
            }
        }
        public System.Management.ManagementScope scope
        {
            get
            {
                return _scope;
            }
            set
            {
                _scope = value;
            }
        }
        public System.Management.WqlEventQuery query
        {
            get
            {
                return _query;
            }
            set
            {
                _query = value;
            }
        }
        public System.Management.ManagementEventWatcher watcher
        {
            get
            {
                return _watch;
            }
            set
            {
                _watch = value;
            }
        }
        public System.Management.ManagementObjectSearcher searcher
        {
            get
            {
                return _searcher;
            }
            set
            {
                _searcher = value;
            }
        }
        public UFDDetect()
        {
            Console.WriteLine("Thread ID is Class :-{0}\n\n", Thread.CurrentThread.ManagedThreadId.ToString());
            ufdDriveList = new List<string>();
            scope = new ManagementScope("root\\CIMV2");
            query = new WqlEventQuery();
            query.EventClassName = "__InstanceOperationEvent";
            query.WithinInterval = new TimeSpan(0, 0, 1);
            query.Condition = @"TargetInstance ISA 'Win32_USBControllerdevice'";
            watcher = new ManagementEventWatcher(scope, query);
        }
        public IList<string> MediaConnectWatcher_EventArrived(object sender, System.Management.EventArrivedEventArgs e)
        {
            Console.WriteLine("Thread ID is Class :-{0}\n\n", Thread.CurrentThread.ManagedThreadId.ToString());
            searcher = new
            ManagementObjectSearcher("SELECT * FROM Win32_LogicalDisk");
            ufdDriveList.Clear();
            foreach (ManagementObject s in searcher.Get())
            {
                if (Convert.ToInt16(s["DriveType"]) == 2)
                {
                    ufdDriveList.Add(s["Name"].ToString());
                }
            }
            return ufdDriveList;
        }
        #endregion
    }
</string></string></string></string>

 public partial class frmHome : Form
 {
   IUFDHelper ufdhelper;
   public frmHome()
        {
            InitializeComponent();
            appPath = Path.GetDirectoryName(Application.ExecutablePath);
            //_UFD.FoundUsb+=new clsUFDrelatedWork.DeviceFound(_UFD_FoundUsb);
            //_UFD.StartDetection();
            ufdhelper = new UFDDetect();
            ufdhelper.watcher.EventArrived += new EventArrivedEventHandler(watcher_EventArrived);
            ufdhelper.watcher.Start();
        }
void watcher_EventArrived(object sender, EventArrivedEventArgs e)
        {
            Console.WriteLine("Thread ID is Main :-{0}\n\n", Thread.CurrentThread.ManagedThreadId.ToString());
            IList<string> driveName = ufdhelper.MediaConnectWatcher_EventArrived(sender, e);

//this line gives the exception 

            toolSubMn1ConfigUFD.Enabled =
                    toolSubMn2UplaodManageFlahsfiles.Enabled =
                    toolSubMn3ManageDownLoadFiles.Enabled = (driveName.Count > 0 ? true : false);
        }
   }
</string>