just take your hdd apart, and then all you need is a sata cable,just connect to to the hdd and plug in the power cable to, go into your bios and enable the second hdd port
take your cable (or a exact matching cable if your physically challenged) cut it directly in the middle , cutback the outer covering to reveal the wires.
the wire has two section (two grounds silver no covering , b-,b+ on one of them and two grounds, A-, A+ on the other ..this is on both ends of the wire u cut in half)
on both of these halves soldier the ends of a length of ribbon cable with 8 lines just rip off the extra and old ribbon cable will do.
MAKE SURE YOUR CONNECTIONS REPLICATE THE CONNECTION THERE WAS BEFORE YOU CUT IT
if ure slick youll use heat shrink tubing after ur soldier cools for a clean finish
the only reason i can think of why you would need a longer cable is a custom enclosure for ur 360 if so remember measure twice cut once ....patch work wiring is UGLY