Is the external drive solely powered by USB or does it have it's own power source?
Anyway, if it is only USB powered, I would suggest a powered USB hub. That way you take any power supplied by the MB/driver/OS out of the equation. Various driver versions may effect amperage supplied and in this case, newer isn't necessarily better.
Or you could try something like this:
http://www.amazon.com/PCD-Universal-...=cm_cr_pr_pb_t
The theory being that you aren't sustaining enough amps to keep your HDD(s) spinning.