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: 
Basically, if I read you correctly, you want a Network Attached Storage device that allows you to access the data stored on it via USB and via an SMB network share simultaneously.

To muse a bit more with you, I think it is possible. It may not actually exist out in the world (yet), but it is possible to build something that behaves this way I think.

There is [this device on newegg](http://www.newegg.com/Product/Product.aspx?Item=N82E16817180005) that seems to do what you are talking about, but judging by reviews it may not do what you want.

If you tried looking around, you might be able to find a way to repurpose a full-blown PC to both provide access to data on an internal hard-drive via both a USB connection and via SMB sharing. However, you might have to be creative with the USB side of things, as I doubt you could have the HDD available as mass storage coming from the PC, due to host/guest issues in USB. You could maybe have the HDD available over USB by using USB as a direct PC-to-PC connection system (kinda like PC-to-PC over parallel port, back in the day).
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>