I have portable Harddrive without a power supply.
It works with a short USB cable but not with cable length more than 1m.
Your HDD is self powered by usb, so the cable length affect the power consumed !
Longer the cable is, bigger the resistance will be. (P = I^2 * R).
You can to the following to solve your problem :
My guess is that your drive needs more power than can be transmitted out of that port down that length of cable.
Get one of those USB leads that lets you use two ports. This should then provide enough power, even if you use a longer lead (with an adapter to connect that to your double one).