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I have this old external hard drive, 80gb, 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 (15cm 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 enought power from the USB? It is connected to the back USB ports of a desktop PC, so I didn't tought it could be that, but now I have my doubts.

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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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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 (support.wdc.com/product/download.asp?groupid=201&lang=en) 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
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