I haven't tried what you're asking, but gut feel is USB would choke pretty quickly. USB 2.0 speed is actually faster than FW400, but the difference is how the data stream is controlled. USB ports need cpu intervention to regulate data flow. Firewire has built-in protocols to self regulate data flow. A Firewire port can run uninterrupted and therefore can pass more data than a USB port.
I think with one Firewire port, you can do everything you're asking. I have a UAD Apollo with a Duo DSP processor. It can stream 18 input channels and 24 output channels plus whatever data that goes to the DSP processor to run plugs. All through one firewire port.
For USB, it's common wisdom to plug only one data-heavy device into a hub at once. IF you had two separate hubs (Most desktop PC's do), you might be able to stream audio in from your interface through one and out to the external drive on another. No idea how many channels would work before bogging down the port.
Unfortunately, firewire is too picky for device compatibility and it seems to be on the way out in favor of USB 3.0 and Thunderbolt.