I seen a problem that the driver for the drive controller on the motherboard was not included in the the kernel. I was able to boot using a USB DVD drive. Still didn't have access to the drives though. I would recommend getting the latest Knoppix build (6.2 I think) and try with that. If the DVD image is too big get the CD image, it should still provide the tools to get the data off the drive if it can see the controller and mount it. If that doesn't work my understanding is that you would have to make a custom CD/DVD with the drivers for your system put into the kernel, but I've never tried that yet.

Other option if your not running a raid setup would be to move the drive to a computer than the version of Knoppix you have can load on.