So think of a block on a CD as some amount of data (like 512 bytes), and some amount of error correction bytes. When the CD is copied those blocks aren't copie

So think of a block on a CD as some amount of data (like 512 bytes), and some amount of error correction bytes. When the CD is copied those blocks aren't copied exactly, but the data is read, corrected and the block is written with the corrected data and error correction data. Most formats don't record bad blocks, nor do consumer writes write invalid blocks. One of the original methods used as copy protection used a special industrial writer write a invalid block, data was ok but the error correction would say it's something different, and check because the consumer hardware wouldn't read or write these blocks.