
If you write a second time to a CD you need to have multi-session capability to
read the second session.  Whether it is photos or just data or whatever.

Here is a simplified way of looking at it:
The first session has the directory structure burned at some tracks.  The
second session has a newer directory structure, but the first directory still
exists, because you cannot change, only add.  So if you put that in a 'normal'
drive and it will only look to the first directory and think it has found all
data.  Multisession kindof means that the drive will look for the last
directory written to the disc, older systems look only for 'the' directory, not
taking multiple directories into account.