Error 11 and 12 are about that very problem: DVD detection issues and 7 is HDD timeout all often due to to IDE cable problems. But if that has been replaced then most likely in this case it is a faulty DVD drive.
this is going to sound kind of backwards, but make sure that the hard drive is on CS, you mentioned that you had changed it and that's likely going to cause problems if you haven't changed it back. it most definitely could cause this problem.
I had a faulty DVD drive on my Xbox I pulled out of storage and was looking at buying a repalcement one. It turns out buying another working system off Craigslist was the cheaper route to go.