Это когда-то пробовали поймать подключение усб модема. Это функция, принимает события WM_DEVICECHANGE которое приходит, в то время когда подключаешь усб. Может поможет.

Код:

void __fastcall TMainFrame::WndProc(Messages::TMessage &Message)
{
   if (Message.Msg == WM_DEVICECHANGE && Message.WParam == DBT_DEVICEARRIVAL)
   {
      ResetEvent(eDeviceEvent);
      DEV_BROADCAST_HDR *header = (DEV_BROADCAST_HDR *) Message.LParam;
      if (header->dbch_devicetype == DBT_DEVTYP_PORT)
      {
         DEV_BROADCAST_PORT *port = (DEV_BROADCAST_PORT *) Message.LParam;
         char wndInfo[512];
         sprintf(wndInfo, "Window  received WM_DEVICECHANGE(DBT_DEVICEARRIVAL) message\r\n\tPort "%s" connected", port->dbcp_name);
         pModemComm->AddToMemo(AnsiString(wndInfo));
                     //WriteFile(hLogfile, wndInfo, iStored, &bytesWritten, NULL);
                     //SendMessage(GlobalData->AppHandle, WM_COPYDATA, 0, (LPARAM)wndInfo);
         hThreadReenum = CreateThread( NULL, 0, TMainFrame::ThreadEnumerate, (LPVOID)this, 0, &dwThreadId );
      } // if (header->dbch_devicetype == DBT_DEVTYP_PORT)
   }
   //TControl::WndProc(Message);
   TForm::WndProc(Message);
}

Detect if a USB device is connected or disconnected?

Код:

unit U_Usb;
interface

uses
  Windows, Messages, SysUtils, Classes, Forms;

type
  PDevBroadcastHdr  = ^DEV_BROADCAST_HDR;
  DEV_BROADCAST_HDR = packed record
    dbch_size: DWORD;
    dbch_devicetype: DWORD;
    dbch_reserved: DWORD;
  end;

  PDevBroadcastDeviceInterface  = ^DEV_BROADCAST_DEVICEINTERFACE;
  DEV_BROADCAST_DEVICEINTERFACE = record
    dbcc_size: DWORD;
    dbcc_devicetype: DWORD;
    dbcc_reserved: DWORD;
    dbcc_classguid: TGUID;
    dbcc_name: short;
  end;

const
  GUID_DEVINTERFACE_USB_DEVICE: TGUID = '{A5DCBF10-6530-11D2-901F-00C04FB951ED}';
  DBT_DEVICEARRIVAL          = $8000;          // system detected a new device
  DBT_DEVICEREMOVECOMPLETE   = $8004;          // device is gone
  DBT_DEVTYP_DEVICEINTERFACE = $00000005;      // device interface class

type
  TComponentUSB = class(TComponent)
  private
    FWindowHandle: HWND;
    FOnUSBArrival: TNotifyEvent;
    FOnUSBRemove: TNotifyEvent;
    procedure WndProc(var Msg: TMessage);
    function USBRegister: Boolean;
  protected
    procedure WMDeviceChange(var Msg: TMessage); dynamic;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  published
    property OnUSBArrival: TNotifyEvent read FOnUSBArrival write FOnUSBArrival;
    property OnUSBRemove: TNotifyEvent read FOnUSBRemove write FOnUSBRemove;
  end;

implementation

constructor TComponentUSB.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FWindowHandle := AllocateHWnd(WndProc);
  USBRegister;
end;

destructor TComponentUSB.Destroy;
begin
  DeallocateHWnd(FWindowHandle);
  inherited Destroy;
end;

procedure TComponentUSB.WndProc(var Msg: TMessage);
begin
  if (Msg.Msg = WM_DEVICECHANGE) then 
  begin
    try
      WMDeviceChange(Msg);
    except
      Application.HandleException(Self);
    end;
  end
  else
    Msg.Result := DefWindowProc(FWindowHandle, Msg.Msg, Msg.wParam, Msg.lParam);
end;

procedure TComponentUSB.WMDeviceChange(var Msg: TMessage);
var
  devType: Integer;
  Datos: PDevBroadcastHdr;
begin
  if (Msg.wParam = DBT_DEVICEARRIVAL) or (Msg.wParam = DBT_DEVICEREMOVECOMPLETE) then 
  begin
    Datos := PDevBroadcastHdr(Msg.lParam);
    devType := Datos^.dbch_devicetype;
    if devType = DBT_DEVTYP_DEVICEINTERFACE then 
    begin // USB Device
      if Msg.wParam = DBT_DEVICEARRIVAL then 
      begin
        if Assigned(FOnUSBArrival) then FOnUSBArrival(Self);
      end 
      else 
      begin
        if Assigned(FOnUSBRemove) then FOnUSBRemove(Self);
      end;
    end;
  end;
end;

function TComponentUSB.USBRegister: Boolean;
var
  dbi: DEV_BROADCAST_DEVICEINTERFACE;
  Size: Integer;
  r: Pointer;
begin
  Result := False;
  Size := SizeOf(DEV_BROADCAST_DEVICEINTERFACE);
  ZeroMemory(@dbi, Size);
  dbi.dbcc_size := Size;
  dbi.dbcc_devicetype := DBT_DEVTYP_DEVICEINTERFACE;
  dbi.dbcc_reserved := 0;
  dbi.dbcc_classguid  := GUID_DEVINTERFACE_USB_DEVICE;
  dbi.dbcc_name := 0;

  r := RegisterDeviceNotification(FWindowHandle, @dbi, DEVICE_NOTIFY_WINDOW_HANDLE);
  if Assigned(r) then Result := True;
end;

end.

Код:

type 
  PDevBroadcastHdr  = ^DEV_BROADCAST_HDR; 
  DEV_BROADCAST_HDR = packed record 
    dbch_size: DWORD; 
    dbch_devicetype: DWORD; 
    dbch_reserved: DWORD; 
  end; 

  PDevBroadcastDeviceInterface  = ^DEV_BROADCAST_DEVICEINTERFACE; 
  DEV_BROADCAST_DEVICEINTERFACE = record 
    dbcc_size: DWORD; 
    dbcc_devicetype: DWORD; 
    dbcc_reserved: DWORD; 
    dbcc_classguid: TGUID; 
    dbcc_name: short; 
  end; 

const 
  GUID_DEVINTERFACE_USB_DEVICE: TGUID = '{A5DCBF10-6530-11D2-901F-00C04FB951ED}'; 
  DBT_DEVICEARRIVAL          = $8000;          // system detected a new device 
  DBT_DEVICEREMOVECOMPLETE   = $8004;          // device is gone 
  DBT_DEVTYP_DEVICEINTERFACE = $00000005;      // device interface class 

//...

private 
 procedure WMDeviceChange(var Msg: TMessage); message WM_DEVICECHANGE;

//...

implementation 

procedure TForm1.WMDeviceChange(var Msg: TMessage); 
var 
  devType: Integer; 
  Datos: PDevBroadcastHdr; 
begin 
  if (Msg.wParam = DBT_DEVICEARRIVAL) or (Msg.wParam = DBT_DEVICEREMOVECOMPLETE) then  
  begin 
    Datos := PDevBroadcastHdr(Msg.lParam); 
    devType := Datos^.dbch_devicetype; 
    if devType = DBT_DEVTYP_DEVICEINTERFACE then  
    begin // USB Device 
      if Msg.wParam = DBT_DEVICEARRIVAL then  
      begin 
       // Усройство подключено
      end  
      else  
      begin 
       // Усройство отключено 
      end; 
    end; 
  end; 
end; 

procedure TForm1.OnCreate({...}); //!!!!!!!!!!!!!
var 
  dbi: DEV_BROADCAST_DEVICEINTERFACE; 
  Size: Integer; 
  r: Pointer; 
begin 
  Result := False; 
  Size := SizeOf(DEV_BROADCAST_DEVICEINTERFACE); 
  ZeroMemory(@dbi, Size); 
  dbi.dbcc_size := Size; 
  dbi.dbcc_devicetype := DBT_DEVTYP_DEVICEINTERFACE; 
  dbi.dbcc_reserved := 0; 
  dbi.dbcc_classguid  := GUID_DEVINTERFACE_USB_DEVICE; 
  dbi.dbcc_name := 0; 
  r := RegisterDeviceNotification(Form1.Handle, @dbi, DEVICE_NOTIFY_WINDOW_HANDLE); 
  if not Assigned(r) then 
   ShowMessage('Error Register Message')
end;