Brucehankins
Since dual boot is not your goal, then the switcheroo of disks (solus=internal win=external) would accomplish your goal of transitioning your main machine.
Granted that hefty 3TB would be the mostly-dormant drive and I can see the waste. So eventually you'd need to do a real PITA real switch (solus on 3TB, win on 320gb).
I don't know why the usb didn't boot when you fired up the windows drive. Maybe someone can answer this. As far as I can see you did everything right and darknetmatrix is right that little bios moves sometimes make the difference, sometimes not.
"invalid signature or no OS present on disk." --I've never seen this. Hopefully someone can interpret for you in the context of the live .iso.