I have 3 IDE hard drives, but only 2 (hda and hdb) are 'seen' by the system.
The CDROM is 'slave' in the 'hdd' position.
There is not a hardware problem, as all my other distros (Caldera, Mandrake, RedHat, Slackware) 'see' and install to, or mount, hdc partitions as normal.
The problem is the same, whether running Knoppix from the CD, or with Knoppix installed on hard disk hda. The console error message when trying to mount an hdc partition is e.g :-

root@dads_pc:/home/dad# mount -t ext2 /dev/hdc9 /mnt/hdc9
mount: wrong fs type, bad option, bad superblock on /dev/hdc9,
or too many mounted file systems
(could this be the IDE device where you in fact use
ide-scsi so that sr0 or sda or so is needed?)

(hdc9 is a good ext2 partition for all the other distros!)

Can anybody suggest what needs to be done ?
Anyway, Knoppix is really impressive !