Well, i'd like to help, but :
  • I haven't got any problem booting with my custom external cable on my test computers
  • I haven't got any USB hub for testing


I suppose you already had a look in the following direction :
  • On HP forums, users reports that HP motherboards usually use a non standard pinout disposition. I also have read all the HP's technical documents related to my HP computers without finding the real pinout definition.
  • I'm sure you know that some motherboard do not supply enough power on their USB to operate an USB HDD which simply don't start. I doubt it is the same because USB dom don't need a lot of power... but was it a powered USB hub or a non-powered one ?


As you can see i won't be of much help here...