Well...this is how I did it:I personally don't know how you could do what you want, 3.5" drives can draw 15w+ when powering up, so even 3x usb ports powering the drive might not be enough. You would need to power the drive from an external power source i personally think, or do the mod i liked to in the post above.
-I've kept the red wire and the black one near him connected to the xbox for the 5v power
-I've cutted the yellow cable and the black one near it and linked them to a 12v2a power adapter (+ for yellow, - for black) and it booted fine

keep up with the good job 