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I am trying to boot knoppix on an old 486 with a sony cdu33a cdrom that runs off a sound card.
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As stonent says, Knoppix can't find the CD-ROM because it's hanging off the sound card. You can buy an IDE (ATA) CD-ROM for about $20-40 these days that will be faster and more compatible -- they are relatively easy to install.

When you DO get it running, you may find that something like IceWM will run more smoothly than the default KDE window manager.