dannyo,
I think you got a totally different USB hard disk from me, mine is really a DIY thhing. First, check your usb hard disk can work properly with the cable on PC, and measure the real current output from the adapter. I got my adapter from a electronics shop, I just choosed the current, voltage and connet port. (5V,1500mA is fine,notice the adaptor output direction)
So, if the cable is good connected and adapter's power is enough, your usb hard disk will function.