The official response I had from WD is below:
**Thank you for contacting Western Digital Customer Service and Support.
As per your previous contact to our services, please be informed that we have further reviewed your case.
Please be informed that the twonky server can only access the Public folder there is no way of changing this.
If you have any further questions, please reply to this email and we will be happy to assist you further.**
I havent had a good look yet, but there are config files that can be tweaked, you can easily find how to access these files, whether you are then able to "see" the usb drive in twonky is another question....
Has anyone been able to do this yet