rob, that sucks.

As someone with a decent amount of 'cutomer service' experience, my advice to you if calling the Loss Prevention number doesn't work, is to go back to the store, ask for the manager and talk to them.

BE A DICK. That is the best advice I could give you. This is the way it works in all retail store/restaurants. If and when the manager blows you off, ask for his/her manager (the General Manager of the store) if they are not there, set up a meeting. This is as high as you should have to take the issue. Trust me, the bigger dick you are to these people, the faster you get what you want. The nice customers are the ones that get taken advantage of. Here is a sample of what I would say:

me: Hi my name is Matt, nice to meet you.
GM: Hi my name is Jackass, what can I do for you?
me: Well, I bought this great HD, and when I opened it, it turned out to be this huge piece of shit. I brought it back to return it, but your employees called me a liar, and refused to refund my money.
GM: Hmmmm.... I don't think they would call you a liar...
me: What are you trying to say? You don't believe me?
GM: No I'm not saying that, it's just that...
me: WHAT?!!?, Just what?
GM: Well, I...uh...
me: Ok, let me make this easy for you. What are you going to do about this piece of shit HD?
GM: I'm not sure I can do anything for you.
me: Oh really. I think you can, and if you can't, I'm sure your boss can, and he would love to know that your employees call the customers liars to their face. What's your boss' name?
GM: I'm sure we can work something out. Let me take a look at that HD. What did you say happened?

This is how the conversation would go. Really.