Just thought I'd share, as I've been on this warpath for quite some time, and have a few threads on here documenting this.
Core issue / scenario: I want to use standby, my mobo keeps USB devices powered when in standby, my USB devices drain the battery.
Finally, I believe I have the comprehensive solution:
1. I use a Dlink hub which a person much smarter than I figured out how to modify it that ONLY the external power will actually power the hub and any devices attached to it. See details here: http://inov8tr.blogs.com/scooby/2006...b_solutio.html .
2. I use a Carnetix 1900, which has a secondary output, and to which you can add the P5V as a third output. I purchased the P5V but soon found that it will drop power to anything connected to it when the ignition is turned off, while the computer is still running (i.e. just starting its shutdown sequence) which resulted in the GPS not connecting properly upon restart and erratic USB behavior in general. So...no good.
3. I've powered things off of Molex connectors from the HD power cables before, but was having some issues (didn't get root cause, unfortunately) when I reread the documentation for the P5V and saw that the on/off switch is just a single wire which needs between 5-12v. So, I finally made the move and instead of triggering the P5V off of the 1900, I soldered on wire and ran it to a +5v line off of the mobo. I put back the original jumper on the 1900 for it to follow DLYON
Now, everything works like a charm, and I can safely say that I'm putting my USB / standby woes to rest once and for all.
It's not rocket science, but perhaps this will help someone else.