虚拟光区映射文件后，怎么使卷标改变？

我在应用程序写了如下代码，但不行：
DWORD dwRec;
DEV_BROADCAST_VOLUME dbvol;
long lRet;
memset(&dbvol,0,sizeof(DEV_BROADCAST_VOLUME));
dbvol.dbcv_size = sizeof(DEV_BROADCAST_VOLUME);
dbvol.dbcv_devicetype = DBT_DEVTYP_VOLUME;
dbvol.dbcv_flags = DBTF_MEDIA;
dbvol.dbcv_unitmask = 8;//I：

dwRec = BSM_ALLDESKTOPS|BSM_ALLCOMPONENTS|BSM_APPLICATIONS;
lRet = BroadcastSystemMessage( BSF_FLUSHDISK,&dwRec,
WM_DEVICECHANGE,DBT_DEVICEARRIVAL,(LPARAM)&dbvol);