As gaxprels said, you have to unmount the cd from the mount table by issuing his/her said 'umount /mnt/cdrom'. This will unmount the cdrom from the device (/dev/cdrom say), update /etc/mtab (that's the file on RH, should be the same for Mandrake), and now the mount point (/mnt/cdrom) will be empty of all cd-rom related data.

There is some other things about umounting that I've included so you don't have to reboot your machine.

# mount /mnt/cdrom
# cd /mnt/cdrom
# ls

<random files here>
# umount /mnt/cdrom
umount: /: device is busy (or some generic message like this)
# cd / (get out of the /mnt/cdrom directory)
# umount /mnt/cdrom
# eject (this should now eject after the umount)

You can't eject a cd if the drive is mounted, and you can't umount a file system if A: you're currently sitting in the mount point, B: if there are any open files or file locks on the file system, or C: if you're using something like HP with Oracle and have a cdrom pfsmounted and you regularly 'umount' it, it won't unmount, you have to pfsumount it. Weird shite...

Hope this helps you on your trek to expertise in the mount/umount field!