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I've recently bought an external hard drive enclosure and it had arrived with an USB cable, with one USB connector on one side and two on the other. Why do some USB cables have two connectors while others do not?

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It's because of power consumption. There is a limit of how much power a single USB port can provide. It is measured in "unit loads" which are 100 mA for USB up to 2.0 and 150 mA for USB 3.0. A single port can provide at most 5 unit loads for USB 2.0 and 6 for USB 3.0, if there are no other devices connected. Some ports may provide more, but they aren't standard compliant and manufacturers can't rely on them. With two cables, device may use at least two unit loads, so it will have enough power to work.

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    To add to this, try using just one and you'll see that either it plain won't turn on/be detected, or (if your operating system is designed for this of course) you'll get an error that there's insufficient power for a connected USB device. – Shinrai Sep 29 '10 at 14:20
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    The USB hard drive does turn on with one USB cable, and can as with two. That was why I was quite confused and had been wondering if using two cables would necessarily mean an increase in data transferring speed. – JFW Oct 1 '10 at 15:01
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    @JFW There probably won't be any increase in data transfer speed. Since it has only one connector on HDD side, it means that there is one USB port on HDD side. Unless thay are attempting to make HDD appear as some sort of fancy RAID over USB (which I think is unlikely), there shouldn't be any speed increase. As for turning on, that's normal. It could happen that your computer has extra power needed to run disk available and assigns it to the port which the disk is using. – AndrejaKo Oct 1 '10 at 15:15
  • So can a device with this connector be powered by two separate cables? For example if I want to connect it to a Raspberry Pi which can't handle the full load, could I use a separate USB cable to provide additional power from a wall adapter? – Metamorphic Apr 9 '19 at 1:57

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