We have a backup disc back then and after the first mission, it stopped loading. So we have another one. It works and we were about to finish Act I. But after the helicopter crashed, the loading stopped! So I had an idea to put the other disc that we have first and load our game data since it's already passed the 1st mission. It worked! BUT it also stopped exactly where the OTHER disc stopped loading! Weird because its a completely different disc.

What might be the problem here? Would transferring the game to a usb work?