If it is an old computer, it will never work for one reason:
The computer has USB v. 1.2 (or something like that, can't remember it correctly) and the hd has USB v 2.0 (otherways it goes way to slow).
Your old USB ports can't handle the speed. You can solve it by going to a computer shop, and ask if they can build in a USB v 2.0 slot in your computer (don't buy a USB v 2.0 hub and set it at an USB 1.2 slot, it won't work!). But even better, buy a new computer!
If your computer is new, you might try other USB ports. Or, is your HD fromatted at the NTFS filesystem? Is your computer linux or mac, or an old version of windows? Than it doesn't work and you need to format your HD (you loose all your files) to FAT32.