So USB boot + X829 (with two SSDs) + X735 works if the X829 is powered through X735 5v out. If the X735 is not connected then booting from USB does not work. Power appears to come through USB 3. But either power is supplied is too little or too late to do USB boot. With power from X735 the USB devices have enough power and early enough so that it can boot.
This brings the question about the X829 case and the cases fan. The video shows the fan being powered from the X735 and the X829 being powered through the USB. From the test above this will not allow USB boot. So the question is can the other 5v line on the X829 be used to power the fan? If this is done will there be any issues with power (5v and amps)?
