command/script for finding *non*duplicate files?

[Adam Funk]

I'm familiar with the duff & fdupes commands, but I'm looking for a
way to find files in one directory (& its subdirectories) that are
*not* duplicated in another one.  I have all --- I think! --- my
digital photos in subdirectories of '~/Pictures' (also backed up
off-site using rsync), but while tidying up the office, I keep finding
various CDs of photos, & I want to check that they don't have anything
"extra" before I throw them out.  The files may have been renamed, so
I want the kind of duplication-checking that duff & fdupes use.  So I
want something like this:

funny_command  -x /media/adam/CDROM -y  ~/Pictures

to list any files under the -x path that are *not* duplicated under
the -y path.

Any ideas?  (I tried googling 'find non-duplicate files' & things like
that, but it just keeps telling me how to find duplicate ones.)

Thanks,
Adam