The current version of free Virtual Box still doesn’t give a support for access your USB. But you can make a trick. Just use the ‘Shared Folder’ to mount your USB to your VirtualBox.
So, you can access the USB from both your Ubuntu and your other OS in the VirtualBox at the same time.
Step 1. Mount your USB to Virtual Box ‘Shared Folder’
Step 2. Access the USB drive from your Virtual Box