|USB cables should be able to maintain the signal up to aboiut 16'. Longer than that and you should use a USB repeater cable. But since yours is only 6' the length shouldn't be a factor. |
Are you using good quality cables? Some cables say they're rated for USB 2.0. I don't know if there's any major difference but if so I assume they are higher quality with better gauge wires.
Does the external drive have its own power supply or does it rely on power from the USB port? If it's powered through the port you should definitely connect it through a powered hub. And it's probably a good idea to use a powered hub even if it has its own power supply if only because there's not much else you can try.