Вы уж простите, что на сях... сподручней было :

//заблокировать
void CMFcDlg::OnBnClickedButton1()
{
HANDLE hDevice = CreateFile ("\\.\E:",
GENERIC_READ,
FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL,
OPEN_EXISTING,
NULL,
NULL);
DWORD dwBytesReturned = 0;
PREVENT_MEDIA_REMOVAL pmr = {TRUE};
if(!DeviceIoControl (hDevice, IOCTL_STORAGE_MEDIA_REMOVAL, &pmr, sizeof(pmr),
NULL, 0, &dwBytesReturned, NULL)) AfxMessageBox ("Door can't be locked");
CloseHandle (hDevice);
}

//разблокировать
void CMFcDlg::OnBnClickedButton2()
{
HANDLE hDevice = CreateFile ("\\.\E:",
GENERIC_READ,
FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL,
OPEN_EXISTING,
NULL,
NULL);
DWORD dwBytesReturned = 0;
PREVENT_MEDIA_REMOVAL pmr = {FALSE};
if(!DeviceIoControl (hDevice, IOCTL_STORAGE_MEDIA_REMOVAL, &pmr, sizeof(pmr),
NULL, 0, &dwBytesReturned, NULL)) AfxMessageBox ("Door can't be unlocked");
CloseHandle (hDevice);
}