You can't use a USB because you have to solder many wires with resistors and diodes to the physical motherboard of the Xbox 360 for the JTAG hack to even work. After you do that you have to dump the NAND, check it, install JTAG software, and xell, and flash NAND back to the Xbox 360. There you have a JTAG.
All threads/videos/pages you see claiming this are absolute fakes.