dselect problem - unable to mount cdrom

[Godric <godric@dial.pipex.com>]

I've just tried to load some packages using apt from a file entry (file:
/cdrom etc.)and got the following error message:

     mount:  /dev/cdrom already mounted or /var/lib/dpkg/methods/mnt 
busy

     Unable to mount /dev/cdrom

Dselect exits without installing packages. Using the umount command as
root brings the message, 'not mounted' . I tried using fuser -muv /cdrom
in desperation and it brings up a whole list. Is there any way to
*unmount* the cd-rom  (and so re-start dselect) other than re-booting? I
had this problem once before, and re-booting fixed it, but was wondering
what's happened and what I did wrong.