Stephan:
I looked at your photos, I can't do math in my head, but ***possibly
your "L" partition was set up inside one of the other partitions In my
OSX & linux dual boot set up, I use the OSX disk utility to set aside a
certain volume of "free space" . . . formatted as fat32 if possible,
Steve:
It appears that you created your Lubuntu partition as an NTFS
file-system, which is why Windows can 'see' it.
In the past, it would not work to install a Linux system in an NTFS
file-system (or FAT32, for that matter). It may be that has changed (I
haven't tried it for probably over