Pressed discs are generally pretty good (as opposed to hgh speed burns on dye discs that you get from sellers on the electronic bay of thieves, which certainly will cause this problem). What you describe could be a bad disc but it might also be caused if booting an older version of Knoppix that didn't know about SATA drives on newer hardware with an SATA optical drive.

I'm surprised that neither disc has the version of Knoppix marked on the top of the disc. And I'm surprised the books don't say. I don't know what versions they are, but you might find out just by booting them on older equipment that they support fine.

As to current versions, as of the date and time of this post, Knoppix CD is at 6.0.1, Knoppix DVD is at 5.3.1. There is a "Get Knoppix" link at the top of this page that will give you links for downloading. I very strongly suggest that you use the BitTorrent protocol to download rather than getting files from a mirror (about 1/2 of my mirror downloads of Knoppix were bad before the torrents became available and the torrents are much faster). I also suggest that you click on the "Documentation" link near the top of this page and read the wiki downloading faq for very important information about downloading, verifying, burning and booting.