Will customizing USB pinout help with backfeeding issue?

Question

I am trying to make my Pi (B) work as both Wi-Fi router and NAS. Setting up software for that isn't a problem for me. But all comes down to connecting USB devices and power supply issue.

Apparently USB HDD that I want to use requires separate power supply, otherwise when I plug it into Pi it does not detect disk (although I can see LED on it saying that HDD is powered up). Original cable that came with HDD box has 3 USB connectors with one being for power supply, but when I plug one end to power supply and two other to HDD and Pi (already powered from micro USB), Pi then restarts and doesn't seem to boot at all. If I do not power Pi from micro USB and connect everything, Pi boots from SD only seemingly thinking that connected HDD cable is for power only.

My question - is that possible if I solder my own USB cable where 1 end would go to wall plug (high quality 5V/1600mA), another end would go to Pi micro USB, 2nd end would go to HDDs USB to powering pins only (+5V/GND) and there would be a bridge between Pi's and HDD's +D/-D (see schematics at the bottom)? Thus I could use one wall plug and keep everything powered without backfeeding power from HDD to Pi. But I am no electronics specialist and not sure if USB will allow such tricks.

               +5V/GND
Wall plug ------------------ PI
                  |          |
                  |          |
                  |          | +D/-D
                  |          |
                 HDD ---------

PS: cutting +5V line from HDD cable does work. But my Pi B rev.2 also works on same cable with drive if I use better power supply. In my case 2.1A supply was good enough to power on everything from same HDD cable.

Answer 1

I would recommend a powered hub. If you do not have one which does not backpower. Even if you do not use a hub you could cut the 5V line to the Pi, or use an adaptor which prevents backpowering, such as I do https://raspberrypi.stackexchange.com/a/13620/8697

PS I consider the omission of an isolator on the Pi Rev B to be a design problem.

Answer 2

It really looks like a powering issue. Do not forget that HDD has a motor, it draws significant current when initiating rotation.

Your power source seems a little bit weak for this task. Knowing A RPi will draw 5W (5V, 1000mA) without any other load there is only 600 mA left to operate your HDD. Of course it depends on HDD specifications, but you better not using a power source at its limits if you plan to operate it a long time.

The wiring you describe should work but I would not recommend this kind of home-made wiring unless you know exactly what you are doing and will use proper tools and materials.

Anyway, You must at least ensure that:

• Your power supply is properly calibrated with respect to the global load connected on it (a 1.5 coefficient should be the minimum);
• Your power supply has a thermal protection (power source must break if overloaded instead of burning);
• Power Supply Common is properly shared to all devices that are connected together (every device has the same voltage reference);
• All you device are sane and no one is drifting or deforming 5V line (5V are guaranteed and no brown out occurs);
• D+/D- is properly shielded (no antenna effect when transferring data);