I'm happy with the USB powered device concept - I have mains powered drives, single usb powered and twin usb powered. I know that when I inadvertantly connect the twin lead drive in the wrong order, ie drive end, then one of the PC ends, it immediately comes up saying 'too much power - usb port disabled' or something similar.
My real question was not about that end of things but whether you could draw power (and/or how much) from a pci slot, ie how does the power get from the MB to the usb socket using one of these pci-based devices.