I was not sure whether to post this here or rather under "feedback and requests", but finally decided to post it here.
I am struggling to come up with a way to cleanly power the Pi and a powered USB hub from a single power source.
1) Powering the Pi with a power source and the hub with a separate, isolated, power source would work. However, this seems ridiculous, since it means adding two power sources with corresponding losses (and costs) to power this otherwise quite energy-efficient system.
2) Powering the Pi from a Hub, as is recommended in some posts, inevitably creates ground loops (there would be two ground connections over the two USB cables), which really should be a no-go from an electronic point of view (BTW, there would also be a loop in the VCC line). I guess this is the reason for many systems becoming unstable.
3) One could fix 2) by interrupting the GND and VCC lines in the USB data cable or the hub (but leave them in the USB power cable). This would avoid the ground loops, but what happens if one unplugs the power but not the data cable? I guess this would break the input circuitry of the D+/D- lines of the Pi's USB port. It further requires messing around with the hub.
What would work better is to power the PI and communicate over *one single* USB port, but it seems this was not foreseen with the low-current polyfuse installed on the regular USB port.
Overall, this leaves me puzzled and I wonder what power configuration the designers of the Pi had in mind.
Any advice is appreciated