My HDD my passport wont connect to my PS3

As far as i know having the two USB cables does supply the HDD with extra power, when you have both USB cables plugged into the PS3. This is because its taking power from two USB ports instead of one. I tried it with the cable i bought from ebay, and if you put only one of the cable ends into the PS3 it has the same issue as if your using the cable that was supplied with the product. Im uncertian how this works, but it just does.