In order to make a USB device available to a Virtualbox guest, you need to have the device physically plugged into the VMhost, (the machine running Virtualbox Server) but it CANNOT BE MOUNTED. (I screamed that last part because it’s important.)
To do this, you’ll need to use the Windows diskpart command to disable automatic drive enumeration on a Windows server installation with the following commands:

diskpart
automount disable
automount scrub
exit

Now, reboot your VMHost, and Virtualbox will allow you to capture and use the USB device in the guest OS. And Robert remains your mother’s sister’s brother. ;)