This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
You are most likely running into copy protection. The game publisher has
placed "unreadable" sectors on the disc, in an effort to confound disc
Instructions for making "backups" of copy-protected games can be found
on the web. See also section (3-39).
If you don't believe the disc is protected, then it might simply be dirty
or scratched. You can try to clean the disc -- use a lint-free cloth,
wiping from the center out -- or see section (7-12) for notes on scratch