CD’s can be burned as multisession CDs. One advantage is that you can mark files in the previous session as “deleted” or changed so that you can effectively use them as a backup solution. Only the newer session is seen by the user when he/she mounts the CD.
To gain access to files on older sessions the fastest way is to give a specific option to the mount command:
mount /dev/cdrom /mnt -t iso9660 -o session=0
Afterwards you can simply browse the old session without any trace of the changes of the newer session.