I think by memory from last summer when I tried a number of live CD's, that Knoppix is specifically designed not to mount the HD. That's why it's called a Live CD. Is there an icon for the HD? If so, try right-clicking on that icon, and see if there is a menu comes up which allows you to mount read only. If so, then once mounted, next right-cllick will let you change to write mode.

Since this is by memory and I did a number of live CD's, it may well be that I am thinking of another Live CD. Try it for yourself.

If you have already tried this, please accept my apology for misunderstanding you.