Actually, this is not a problem with SMB itself, but with USB mount points appearing and disappearing, and not being assigned consistent names.
See this thread for more info, it will explain to you what is happening:
http://forum.mikrotik.com/t/mount-point-for-usb-storage-is-not-consistent/76897/1
So this presents itself as a SMB issue, but its actually an underlying USB port mounting problem.