I know these posts are a little old but I have been battling with my small 40 gb ext drive getting nowhere with my new PC and yet it would work perfectly on my lap top XP syst.
But recently a mate of mind brought a similar drive to get my photos of an outing we did together.
I had the same problem with his drive on my PC.
He saw I had it plugged on a cable in turned plugged into my 4 usb ports. and told me to try it directly on the pc usb port.... It worked! And so did mine later on!
So after reading all these posts and all these well thinking help but none of them mentioned the fact long cables and indirect connections can be enough to prevent your ext. HD to be recognised.
USB hubs are so practical no need anymore to crawl to access the pc ports, but in this case they were the problem!