Well, I know this may seem like the hard way, but it's a step I take as well. You said that as soon as you unplug your external hd from the usb, all is well. Boot up without the external plugged in, and burn all of the images you want to save onto cd or dvd. On another note, don't buy a seagate, I've used their drives before, and never again, why, same problem you are having now. Too much hassle, I know it's a pain to go out and buy another hd, install it, etc, but it's worth it. Another recomendation is once you have everything fixed, put in a second internal hd, and keep a copy of all of your images there as well. Something of note, this almost sounds like a driver conflict with your external hd. The external may be ok, and what not, but there may be a driver conflict somewhere. I have an external as well, love it, but sounds like this may be the source of your problem. Good luck Bobby.
I know that I am one with beauty, And that my comrades are one. Let our souls be mountains, Let our spirtis be stars, Let our hearts be worlds.