Currently, USB support in VirtualBox relies on a deprecated mapping between VirtualBox and the Linux USB system. This results in errors saying "Failed to create a proxy device for the USB device."  The solution at this blog is to run the commands:

If VirtualBox USB access is impossible with mass-storage devices etc, enter the following in a terminal:

VBOX=$(grep vboxusers /etc/group | sed 's/vboxusers:x:\\(.*\\):.*/\\1/')