The very first implementation of my Odroid Server ran off an 8 GB SD card. While its not a problem to find a minimized Linux distro (Ubuntu Mate comes in full and minimize) it was still a very small amount of space available after installation. Just running my Plex Server was a pain, as thumbnails and other items would quickly fill the remaining space I had. I was becoming stressful, and I was about to abandon my server until I could buy a bigger card.
Well to my surprise Linux supports symbolic links. Very simply, a symbolic link allows you to create a shortcut, that is a path to a separate directory or file. A good example is my current Apache server that is installed on a 32 GB SD card. To access website folders, the path is /var/www/html. I want to store all my websites on an external drive (at this point I have like 15 GB remaining on the SD Card). So I have my web folders on an external /media/storage/ and a folder named test_site. I can create a link to test_site inside the html folder. This allows me to use the path /var/www/html/test_site/ and access that website as if the folder was stored on the SD card.
Here is how it's done in terminal
sudo ln -s /media/storage/test_site /var/www/html/test_site