Oh right, did not think about this.
What about this event?
Code:
EVENT_MOUNT_FAILURE (number eventCode, MountFailureReason reason, number arg1)
Maybe it triggers if you cannot mount and the
MountFailureReason provides infos then?
-> MOUNT_FAILURE_REASON_INVALID_LOCATION
Edit:
After rethinking this you'd need a function which actually tries to mount, and this is private if I rememmber correctly :-(