problem with usb devices in Lenny - Virtualbox 3

[Tuan Nguyen <duyatuan@gmail.com>]

Hi all,
I have added the following line into fstab 

	none /proc/bus/usb usbfs auto,devgid=1000,devmode=0664 0 0

When I first boot the machine, Lenny can mount it normaly, whenever I
start VB with XP guest, the device disapears in Lenny but I can access
it via XP guest.
though I've tried to remove the usb flash disk in XP and uncheck it in
VB's USB devices list, Lenny doesn't mount it again (or show sth like
USB Flashdisk in Places/Computer for me to mount it manually).
so I ran "mount -a" and get the error msg 

	mount: none already mounted or /proc/bus/usb busy

Please help me out! thank you!