This is interesting. I'm currently looking after 4 SME Server machines, on a wide range of different hardware. I've not ever seen SME mount a USB disk.
When I plug a USB mass storage device (be it hard drive or flash drive), SME creates a /media/<diskvolume-name> directory. I then mount it manually, and unmount it manually when I've finished with it.
Never having read the section on USBdisks, I thought this was design behavior. Its worked ok for me, though in one instance, a server box became confused and would not create the necessary directory until I bounced it.
If it really is supposed to automount, I guess I'd better report it as a bug, since it never has for me...
paul.