Why are read errors occuring?

Why is it that when you copy AVI files onto a blank CD or DVD without putting them in a folder, you cannot copy them back to HHD.

I used to have this problem when I first started encoding DivX movies and then just purned them to CD. However after battling to them copy them back to PC I started putting each movie in a folder and never had problem again.

However this weekend someone gave me a DVD of some TV episodes he had burned to disc and they were just burned to disc. I tried everything to make ISO’s of disc and could not, getting read errors. Tried Alcohol 120% , DVD Decrypter, Unstoppable copier even Windows explorer manually.

Why does this happen ?