Ah, sorry. My misunderstanding.
I think I may have a possible solution for you then.
Have you tried mapping it through your network? (i.e. bouncing the data off your hub/switch.)
That way when you try to create the network install image you can specify the drive by using a UNC path, rather than trying to use a drive letter.
I seem to remember reading somewhere in the documentation that the Network Deployment Wizard does not like you to create the image on a mapped drive but prefers the full UNC path to install to. So, using this logic, if you can give it the UNC path to your USB Drive then it may solve the problem.
Hope this helps.
Cheers,...Jon.