Problem with Dead Space 2 disc 2?

Ok so I bought dead space 2 yesterday. Started playing it, when I got to chapter six, after I kill the brute and what not, I get on the elevator to ride up to the solar panels. The game says "insert disc 2" So I open the tray, insert disc 2, and it loads up and says "chapter 7"....then nothing happens! I wait, and wait, and wait....and WAIT on the elevator, it never opens up, it never stops, it just keeps going, I sat there for like almost 10 minutes. So I paused and tried to return to my lasy checkpoint and I sit at the loading screen for forever! So I figured maybe the disc was scratched. So I clean it, kill the brute go back to the elevator and nothing happens! So then I paused the game and tried to quit and I get the xbox error message that says the disc is unreadable, clean it off yada yada yada. So I loaded up another file where I'm on chapter 11, and it works fine! So I thought maybe the file was corrupted, but to be on the safe side I returned the game to wal-mart and got another copy. Came home, deleted the file, started a new one this morning, I just got to chapter 6, got on the elevator and it still wont work! I keep getting the disc unreadable message. There is no way in hell I got 2 copies of the game and they both don't work. I have the file saved on my hard drive, maybe that's the problem? I also have a memmory card in, sooooo.....???? Can anyone help me? thankyou in advance.