Weird disk problem...

So, for a little backstory… I got Halo reach a long time ago and broke it, well my friend broke it. So, last week I put it in my Xbox and it worked fine, surprisingly (this is a newer model of the 360, so) But i had two copies of Reach so i gave my friend the old, scratched copy, and to my surprise it didn’t work. So, i gave the new copy to my friend and it did work. Everything is solved now, my friend has the new copy, and i have the old, but i am still confused HOW???
Is it the disk drive…
Is it the newer model…
Or is it somehow the disk?