i have windows 7 ultimate 64 bit while connecting external hard disk directly to usb port its working but when i connecting this hard disk using logitech 4 port usb hub on the same usb port its not detecting but my pen drive is working through usb hub.
You might need to provide power to the hub. What else have you got in it? I've got a few different hubs myself and if I plug more than two hard drives/usb keys in it, they cease to work. I'd say it's probably a power problem.