[TriLUG] why does usb card on PCIe bus need extra power to detect a disk?

[Brian Henning via TriLUG]

> I'm wondering why this PCIe card is recognised by the kernel, but needs power for the 
> external usb disk to be detected. The external disk has its own PS. I had assumed that 
> only signal was going over the usb cable from the computer to the external disk. If 
> some power is needed from the PCIe card for the external usb disk, why isn't the card 
> getting it from the PCIe bus?

I can only make some weakly-educated guesses:
 o  Perhaps the USB interface chip inside the external HDD is itself bus-powered?
 o  Perhaps the PCIe card is designed not to supply port power from the PCIe bus, 
    since it's got the Molex power connection?
 o  Given the expectation of power on the Molex connector, perhaps the PCIe card
    has safeguards that shut off the ports if the external power isn't connected?

The 0.1" header is probably for connecting to USB jacks built into the computer's case.

> I assume the usb 3.0 hub on the mobo is still talking to the PCIe bus.

I wouldn't make that assumption.  That said, I find that limitation surprising; even a
single PCIe lane can clock at 16 Gbit/s, which strikes me as more than enough to serve
even a 10 Gbit/s USB3.2 connection, even considering the (up to) 20% overhead in PCIe 
signal coding.  *shrug*

-B