does mandrake use a /dev/cdrom symlink ? if so, does it point to /dev/hdc ?
in that case, just let point /dev/cdrom to your cdrom /dev/scd0.
you can also try /dev/sr0 instead of /dev/scd0, which should be the same device.
if mandrake does not use a /dev/cdrom symlink, try changing the cdrom entry in your fstab; should be something like:
/dev/sr0 /<cdrom mount point> iso9660 ro,user,noauto 0 0
/dev/scd0 /<cdrom mount point> ...
if you tried that already, or if it's some rpm-drake or mandrake specific problem, just ignore my ramblings