IFSMgr_UnmountChildVolume( unsigned long VRP, unsigned long Drive )
This service is used to unmount a previously mounted child volume and remove its mapping to the host volume. Note that the host volume is not unmounted, it still remains valid. The host volume is unmounted only when the host drive itself goes away.
VRP
Supplies the system volume handle for the child volume.
Drive
Supplies the 0-based drive number for the drive unit the child is on.