This is exactly the same thing that happened to a friend of mine with CompUSA several months ago. Laptop went in, came back with someone else's hard drive.
Long story short, there was information on the drive he was given that got CompUSA into big trouble (health records, some interesting pictures) and he ended up getting about a $7,000 settlement out of it.
This is an ongoing problem with CompUSA. Go complain directly to the manager of the store and threaten legal action if necessary, this kind of complete incompetence should not be tolerated.
Firstly, CompUSA was nothing but a middleman here. Apple is at fault.
Secondly, threatening legal action is a really bad move when dealing with CompUSA, it should only be a last resort. In most cases the people at the local store will do their best to help you out, especially if they are at fault. But if you threaten legal action, they are required to cease interaction with you and refer you to the corporate office. You don't want this because CompUSA corporate is the physical manifestation of incompetence. The best thing to do if you don't think you're getting anywhere with the local store is to file a corporate complaint. This will get back to the General Manager of the store very quickly, and will put a lot of pressure on him to resolve the issue.