Under Solaris, umount come does not allow a file system to be unmounted if a file on the file system is busy i.e. open by other programs/process or by user. You will get device busy error. To get rid of this message you can pass -f option to umount command. Suppose you would like to forcefully unmount a /cdrom/cdrom0 then you can type following command (login as a root user):
# umount -f /cdrom/cdrom0

Please note that using this option can cause data loss for open files; program(s) which access files after the file system has been unmounted will get an error.