You're probably just looking in the wrong spot. Aside from a hard drive failure you should have no problem retrieving any data.

If your hard drive is making strange noises when you try to start it up, then you could have a problem. Knoppix should pick up the hard drives on your desktop screen. If you have an internal hard drive and two external hard drives then you should really see 3 hard drive icons on your desktop.

Try disconnecting your external hard drives and start knoppix again, it should pick up the hard drive and be the only drive on your desktop to access. I've used it a couple of times to get me out of some hairy situations.