**What John found out by using the shorter cable is more likely to be solving a Voltage-drop problem rather than Current.
It may not be the same situation for all, though, due to differences in motherboard design out there. Some, if not all, will likely experience the Current drop situtation, which should be handled by the provided double-plug cable when hooked up right.
There may even be some few machines that provide tawdry USB power -- and suffer from both Voltage and Current drop. That would be an entirely bad situation, solvable only with external power from either externally powered USB hub or perhaps a wallwart connection to drive if it has a power socket on it.
--Mac