problem solved today (I hope can help you, too).
Since changing /etc/fstab like many people and user's manual suggest to do don't solve the problem, I tried this way:
1 - create a new group called "usb";
2 - locate file usbfs: in my case is /sys/bus/usb/drivers (I suggest to find the file with a usb device inserted;
3 - modify file /etc/fstab inserting a line containing the right path and the number corresponding the "usb" group :
none /sys/bus/usb/drivers usbfs devgid=503,devmode=664 0 0
4 - command mount -a;
5 - start VB and try...;
I can't believe it, but now it works!!!
Please let me know if this way effectively fix the problem or I drunk too much!