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I have this old external hard drive, 80 GB, nothing too fancy, that is powered entirely by USB. The problem is that it keeps disconnecting itself.

I remember that I tested it with a short cable (15 cm, more or less) and it worked fine.

Now I'm using a one meter cable and it keeps disconnecting. And I can't find short USB cables anywhere!

Could be that the hard drive is not receiving enough power from the USB? It is connected to the back USB ports of a desktop PC, so I didn't think it could be that, but now I have my doubts.

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Is it possible that something untoward has happened since you last tested with the short cable? – Daniel R Hicks Jun 23 '14 at 15:24
(One possibility is that the USB connector on the drive is flaky and only makes good contact if the cable is positioned a certain way.) – Daniel R Hicks Jun 23 '14 at 15:27
up vote 0 down vote accepted

A cable that is too long could reduce the power delivered to the device to the point where it does not work. The longer cable will have more resistance than a short one, which creates a voltage drop and reduces the current to power the device. I don't know if the USB spec requires all devices, especially high-power devices like hard drives, to work at the full 5 meter cable limit.

Although I'm surprised it took only a one meter cable to see a difference. I just connected my 120GB WD Passport (1 year old) with a 1 meter cable, and it works okay.

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If the drive or device you are trying to attach via the cable draws its power over the cable: Absolutely yes!

The length of the cable plays a larger role in this case that since it is low voltage power transmission. This is no IT topic, but pure physics.

Testcase: I've just attached two identical external 2,5" 1 TB HDDs via a 3 m USB 3.0 SuperSpeed cable. One worked. The other unmounted itself every few seconds. So the voltage drop was just on the edge, one drive worked, the other (again, identical model) didn't.

Another 1 TB drive model didn't work at all, yet another did. Switched to a 0,5 m cable and all drives works as expected.

(At least) 4 factors play a role

  • The power consumption of the drive
  • The length of the cable
  • The cable quality (= mainly wire diameter) and shielding
  • The output of the USB port (my MacPro front ports offer more power than the USB standard defines, on this ports, the 3 m cable works)

What you can do:

  • Use devices with their own power supply (best solution)
  • Use a high quality or shorter cable (a little gamble)
  • Use an "active" cable if you need a longer one

While the USB 3.0 standard doesn't strictly define a cable length a maximum of 3 m is recommended. Active cables can be longer than that.

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This is a good answer, but it only "suggests" that the cable length is the underlying reason. You could turn it into a great answer by making it definitive with electrical measurements (e.g., voltage at the drive or round trip resistance of the cables' power conductors). – fixer1234 Jun 9 at 0:56
Absolutely, thanks for your positive feedback. I was thinking about measuring the consumption and voltage drop, but didn't have the resources to do so. – awenro Jun 9 at 7:42

The direct answer to the question is no... Unless the cable is damaged.

In practice, the USB specification limits the length of a cable between full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches).

If the hard drive is an old desktop drive (3.5 inches), then USB does not supply enough power to spin the motor. You would need an external hard drive enclosure with its own power.

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Mmm, I bought a cable recently so it's brand new. And it's a Wester Digital Passport ( so no, it's a normal external hard drive. – Zequez Aug 17 '11 at 4:28
have you tried swapping the cable? what about trying it on another machine? the drive and/or cable could be defective. – Keltari Aug 17 '11 at 4:31
I tried with many cables and in many computers, the only cable that worked fine was this one that was 15cm length, but it wasn't mine. – Zequez Aug 17 '11 at 4:59
If you have a powered usb hub, you might try plugging the drive into that. Or just buy a smaller cable to test with. – Zach Aug 18 '11 at 0:49
Your direct answer is factually incorrect. The cable length can play a major role for any devices powered via the USB cable. – awenro Jun 8 at 12:23

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