Wd ext hdd recognised with short usb cab only

Hello,
I have a 250 gb western digital external hard disk that came with a short say about 15 cm usb cable. the hdd works fine with the cable. when i connected it with a long usb cable, say 75cm , purchased at a retail store (original cable) the hdd is not recognised. win xp says new hard ware found and asks for drivers. the hdd is connected directly to usb port on the backside of pc to the motherboard . how does the usb cable length matters for its functioning ?