I have had the same problem, Disk 1 Installed ok Disk 2 stops at 44%. I could still play the game but when a mission ends with disk 2 in the drive it will freeze at the loading screen. I went and got a replacement for disk 2 and the same thing still happens. After looking on the net the problem seems to be related to an anti-piracy update " dae.bin being updated" and has affected other new games as well.
If this problem is not fixed I may as well put my 360 in the rubbish and get a PS3 but not before getting my money back for the game and for a console that Microsoft
has effectively broken.