I always had lots of trouble with usb devices until I stumbled on a forum comment that your pclos platform & your vbox system vie for mounting it. Once I grasped that concept I could manage usb's more easily.
So, after you enable usb in your vbox/settings tab, start your vbox guest & plug in your usb. Your PCLOS device notifier will recognise it, but ignore that or esc it, & click devices in your guest system's vbox frame
& you should see your recently plugged device, tick it & it should mount.
btw, this is with the Sun version, 3.1.6 v59338