From the information you've given, I surmise that:
- You have a black box device which you can communicate with only by plugging a USB mass storage device into it.
- Physically plugging and unplugging a USB drive is not acceptable, you won't have physical access after deployment.
If any of these assumptions is false, you'll have an easier time.
What you're looking for is a way to make a computer appear as a USB storage device. In principle, this shouldn't require much electronics — I think the right gadget could be mass produced for no more than a couple USD plus shipping. But I can't find a reference to the right gadget existing.
Merely using a USB computer-to-computer cable won't do the trick: these simulate an Ethernet network, and the black box doesn't speak Ethernet. And USB-storage-to-LAN converters won't help, they go the wrong way round.
You could design a robot arm that unplugs a USB stick from the black box and plugs it into a computer. Now that the obvious but impractical suggestion has been made...
You could have the gadget designed, and write a driver for it. But this might be out of your price range, especially if you don't need millions of units.
There may be a sophisticated NAS device that can talk to several computers over iSCSI and USB (those last two words are the difficult part).
Some mobile phones and PDAs can act as a USB storage device, and additionally have a GSM, wifi or Ethernet connection. But they often act as storage devices only when switched off, so I don't know if there is one that could be used as a bridge or server.
Here's a discussion about a somewhat similar requirement. The only possible solution it suggests is this USB-storage-to-wifi bridge, which looks like it would be suitable — but it's expensive.
There are USB switches (a.k.a. sharing hubs) that allow you to share a USB device between two computers. You could use one to share a storage device between the black box and a computer. The problem is that they usually require the user to press a button to switch between the two devices — but if you find one where the switch can be controlled remotely, you have a solution.