PS 2 Back-Up Freeze!

I have been backing up my PS2 games and I have just finished Mercenaries. I have the disk swop set for my Slimline PS2. I load the swap, then load the Mercenaries back-up. It starts up and when it shows “loading” in the beginning, it freezes. I tried another copy and same. The original freezes in the middle of game play at times. Could that be the problem? Any ideas? I have succesfully backed-up my other games with no problems. Thanks for any help or ideas.