Why isn't my mounting point visible in Windows Explorer?

When mounting points are added or removed, a system message is broadcast (WM_DEVICECHANGE) instructing Windows Explorer to refresh the list of drives available. However, these messages cannot cross user session boundaries; so if, for example, the application is running as a service, Windows Explorer may not receive the broadcast and thus fail to refresh the list of drives. To address this issue, the product includes a Helper DLL which, among other things, helps ensure that Windows Explorer always refreshes the list of drives regardless of which user session the application is running in.

Also, mounting points can be global (visible in all user sessions) or local (visible only to a specific user session). The AddMountingPoint method creates global mounting points by default; applications must include the STGMP_LOCAL flag in the Flags method parameter value to create local mounting points.

When creating a local mounting point, applications can specify a specific user session for it to be visible in by passing that session's Authentication ID for the AuthenticationId method parameter. If no Authentication ID is provided (i.e., 0 is passed), the local mounting point is created in the current user session; and if the application does this while running with elevated rights, then the local mounting point will only be visible in the elevated session, and consequently won't be available to applications in other sessions (such as, e.g., Windows Explorer).