The ToB CD has a copy protection scheme on it (A first for Bioware AFAIK), and that
always means that some CD-ROM drives will fail to read it. It also means that the protection checking was likley removed in the patch due to excessive complaints from users who couldnt access the disc.
If the problem persists, go to
www.gamecopyworld.com and install a NO-CD patch. Youll need a full install of both games to do this, but at least youll be able to load the game.