I've had a look around the forums and google but I couldn't see anyone else mentioning this problem.
I was previously using an old 40gb hard drive to store my back ups and this was working perfectly, I decided to upgrade this to a 750gb Samsung drive and technically this works fine too but as soon as the drive is plugged it it gives horrendous screen interference. Games work fine on it though as long as you can peer through the static!
I've tried both normal AV and component cables and it still occurs. Does anyone have any idea why this is happening and how to stop it?
My guess is the drive isn't properly shielded. I would consider this a factory defect and if you just recently bought it, see if you can return it and exchange for another drive (same model at first, and hope it doesn't happen again). If it does happen again, then you'll want to try a different manufacturer.