root@pyy / # umount /cd0
umount: /cd0: No such file or directory
root@pyy / # umount -f /cd0
umount: /cd0: No such file or directory
root@pyy / # df cd0
Filesystem  1024-blocks     Used    Avail Capacity  Mounted on
/dev/cd0d        391896   391896        0   100%    /cd0
root@pyy / # ls -l |grep cd0
dr-xr-xr-x   1 root  wheel  688717824 Jan  1  1970 cd0
root@pyy / # mount /dev/cd0d /a
mount_ffs: /dev/cd0d on /a: Device busy

All I did was that I accidentally removed the CD before umount and then the
daily script tried to run find or something on /cd0.

Looks like removable media support could use some improvement.. :-/

  -jm