I make some mistake in the procedure:
1) Starting the Host, with no USB device connected (except for the mouse)
2) Check the USB configuration for the Guest machine: there is two USB device configured
a) the external HDD
b) the flash USB 4 GB memory
Both were added using "specific device" and reviewed putting blank all fields except "vendor" and "product"; for both, the "Remote" attribute was set to "Any"
3) Starting the Guest system
4) Check for the available USB devices: the list show only the items "USB optical mouse" and "Unknown device"
5) Connect the external USB HDD to the Host
6) The host detect the connected device and show it as "F:" unit in the Devices list
7) Again, check for the available USB devices in the Guest system: now I find three items
a) the USB optical mouse
b) the "Sunplus Innovation Technology USB to Serial-ATA bridge
c) the "Unknown device"
Standing the mouse pointer on the b) item, the USB device manager show me the related properties: the status is "Captured", but no new logical HDD unit appears in the Guest Devices list.
8) From the Host system, I eject the USB external HDD
9) OK, device ejected: no more "F:" unit is visible in the Host Devices list
10) Go to the Guest system to view the current USB devices list: nothing change from the point 7) and no new unit appear in the Guest Devices list.
What should I do at this point?