Question: The answer to this is likely "It is not possible.", and from what I know about filesystems and storage, I would say the same thing. But, I thought I would try the great wisdom of SuperUser:

I'm looking for a NAS device that will serve the same content over USB and via SMB.

I have a device (let's call it the reader) which will read files from an external USB drive. I would like to attach a drive, but also make that drive writable across the network. The reader does not have a network port. I get that the reader considers the USB directly attached storage, so it partitions and formats it, while anything that served up the drive's content over the network (via SMB or something) is serving up file content and not a lower level storage device like the USB interface, and you'll end up with two different things having the filesystem mounted, which going to cause trouble.

Answer: 
I dont know if you ever solved this, but have a look at this, I just ordered one, had a similar requirement to you, I want to carry a pc around with me and thats it, and need to be able to easily share the data on it like a drive , such as AV progs or Hirens disc etc with other pc's

<http://cgi.ebay.co.uk/USB-Go-Link-PC-PC-Transfer-Network-Cable-Card-Reader-/120645125445?pt=UK_Computing_CablesConnectors_RL&hash=item1c17028545>
It is not possiblwe. USB disk file-systems are managed by a host computer which is aware of blocks, allocations, which files are open, where they are, etc. It is just a block device.

A NAS on the other hand maintains internal awareness of all of these factors and they won’t match the awareness of the computer that has attached the disk via a USB port.

SANS do this sort of thing, but in different volumes or partitions. iSCSI is the SAN equivalent of USB.