How to enable Time Machine to backup the data ON an SD Card? (NOT backup TO an SD card)
Three things will need to come together for this to work:


The Filesystem on the SD card needs a .fseventsd hidden directory and the system to track all changes (which typically gets created automagically when you format the card as HFS+) I haven't seen anyone hack Time Machine to back up filesystems that don't register with Apple's file system events API.
Make a test backup once you've gotten the filesystem into one that Time Machine supports. You can then check if /Volumes/SD is included in the files for that backup by inspecting the preferences for that backup
If you need to manually include the system, you can add it by changing either the metadata flag that is causing the item to be skipped or by editing the system exclusion lists.


You will need to pick parts of these two questions and answers to find both the settings for a specific backup as well as where the various system exclusion rules are stored and configured.


On OS X, what files are excluded by rule from a Time Machine backup?
Where does Time Machine store its settings?


In short, it might be easier to use rsync or some other tool to copy the SD card contents to a drive that already is being backed up, but as long as you have total control over the drive and the system, you should be able to bend it to your will and get the SD card to be seen and backed up every time it's present and a backup happens.