try fuser, it should show if someone or something is in the directory in question.
so man fuser
eg
[root@vinson etc]# fuser /mnt/ (gives no return)
[root@vinson etc]# cd /mnt/
[root@vinson mnt]# fuser /mnt/
/mnt/: 16337c
[root@vinson mnt]# ps aux |grep 16337
root 16337 0.0 0.0 4268 1372 pts/0 S 08:19 0:00 -bash
root 16408 0.0 0.0 3676 676 pts/0 S 08:25 0:00 grep 16337
So this shows "me" as root in a bash shell in /mnt so /mnt cannot be ejected/unmounted while "I" am in there. This should work for where ever the cd is mounted.
regards
S