Não use o comando dd para copiar imagens de cd

Create a CD/DVD ISO image from disk.

readom dev=/dev/scd0 f=/path/to/image.iso

A explicação vem em inglês - um incentivo para você estudar um pouco, e pra min também :)

Many like to use 'dd' for creating CD/DVD iso images. This is bad. Very bad. The reason this is, is 'dd' doesn't have any built-in error checking. So, you don't know if you got all the bits or not. As such, it is not the right tool for the job. Instead, 'reaom' (read optical media) from the wodim package is what you should be using. It has built-in error checking. Similarly, if you want to burn your newly creating ISO, stay away from 'dd', and use:

wodim -v -eject /path/to/image.iso

Fonte: command-line-fu