The spec varies between USB1.1 and USB 2.
For USB 1.1 the max distance should be 3m. If you are using a hub the max distance does not increase.
For USB 2 the max distance is 5m. You can daisy chain hubs to reach a max distance of 30m.
Wikipedia has a good article on USB including cable lengths.
It may be a power issue. My USB disk enclosure has a separate power lead which I don't need for my disk but I know others who do.
Do you have other power consuming USB devices attached. Try disconnecting all other USB devices so the disk is the only one.