1st Time XBOX Owner - Question Over Disc Error

I just bought a modded / chipped Xbox from a market after purchasing a lot of games ( original ) on ebay, I was told it would be a goofd thing to get these backed-up as well

The original games work fine, however 2 of the back-up games do not work ( where as the other 9 work fine )

I get a message saying ' Please insert the disc into an Xbox 360 console ' then followed by messages in other languages.

I checked the discs in my PC and they seem to bs NTSC, so I wondered if that was the problem, until I checked the other working copies and saw some of them were in the same format.

What do you think the problem may be? Strange how 9 games work fine, but these 2 dont work. Known problem? Problem with them being burnt incorrectly?