The point of the "Model A can boot from USB" sub-thread is that the Model A does not have any LAN/USB-hub chip connected to its (single) USB port on the SOC. All of the B (including the B2) have such a chip present, which prevents the USB from running in "slave" mode.From what was stated, even with the Model A, the initial boot of the GPU and loading the Linux kernel is done from the SD card.
The upshot of all this is that it is, in theory, possible to make the USB port on the Model A work as a slave (I hope I got the terminology right...) and thus to boot and load the OS from another (presumably, a Windows or Linux PC) host machine - using no SD card at all.
Heavy emphasis on the "in theory" aspect of this, because as far as I can tell, no one has actually done it. Except maybe Eben. I seem to remember a post from Dom (maybe years ago) saying that although he knew it was theoretically possible, he'd never actually done it.
And if Dom hasn't done it, there's faint hope for the rest of us...
P.S. All of which is, as I mentioned in an earlier post, OT for the OP, since he wanted to boot from a USB hard disk, not from another host PC.