Are There any Risks in Using Y-Cables with USB Peripheral Devices?

Sometimes it can be confusing when a tech specification ‘prohibits’ the use of a particular accessory item, yet everyone seems to think using the item in question is completely fine. Who is correct in the end? Today’s SuperUser Q&A post has the answer to a worried reader’s question.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader muttley91 wants to know if there are any risks involved in using Y-cables with USB peripheral devices:
I have been advised to use a Y-cable to power a portable USB hard drive from two ports since it is unable to draw enough power from a single port. Are there any risks or dangers of overloading or damaging the hard drive by using a Y-cable this way?
Are there any risks involved in using Y-cables with USB peripheral devices?

The Answer

SuperUser contributors misha256 and Dmitry Grigoryev have the answer for us. First up, misha256:

The USB specification prohibits the use of Y-cables:

Use of a Y-cable (a cable with two A-plugs) is prohibited on any USB peripheral. If a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered.
But the real world said “forget your silly rules” and uses Y-cables all the time. Technically, there is pretty much zero risk of anything bad happening, just do not use Y-cables via an unpowered USB hub. Plug the connectors straight into the computer’s USB ports.
“Are there any risks or dangers of overloading or damaging the hard drive by using a Y-cable this way?”
Using a Y-cable does not increase the voltage, it only makes more current available, so it is all good. The hard drive will take as much current as it needs.
You are far more likely to damage the contents of a hard drive by running it under-powered. There is nothing worse than write operations failing midway through. So I implore you, use the Y-cable.

Followed by the answer from Dmitry Grigoryev:

Y-cables are forbidden by the USB specification for a reason. Connecting the A-plugs of a Y-cable into two different host ports (i.e. two different computers, a computer and a hub, etc.) can lead to multiple electrical problems:

One of the hosts may be unpowered while the other one is powered on. This will result in the powered host pushing current to the unpowered one in the wrong direction, possibly resulting in damage.

Even if both hosts are powered, one will provide a slightly higher voltage than the other, resulting in a problem similar to problem #1 above (perhaps less severe).
If you are extremely unlucky, you may get two computers with poorly isolated PSUs connected to different mains (120V or 240V) networks. Connecting those with a common ground cable (which is a Y-shaped USB) will zap both computers instantly. Though in this case I would be more concerned about electrical safety in general rather than about Y-cable usage.

If you make sure to connect both A-plugs to the same USB host, using a Y-cable is fine (I have one myself). Do not worry about over-powering the drive, it will take just as much current as it needs.
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