How to create a FAT16 partition on an SD card with OS X El Capitan? I want to prepare an SD card for use in an old camera using CHDK. The card needs to be FAT16 formatted. This doesn’t work from Terminal anymore, maybe already for years. Disk Utility provides ''FAT'', but this is FAT32. I have no other (Windows) computer. What do I do?
You can format any drive to FAT16 with newfs_msdos Command

1) Launch Terminal

2) Find The Drive you Want to Format

#mount
/dev/disk2 on / (hfs, local, journaled)
devfs on /dev (devfs, local, nobrowse)
/dev/disk4s1 on /Volumes/USB_Disk (msdos, local, nodev, nosuid, noowners)


In this case, my disk is disk4.  Your disk will most likely be different!  Be sure to reference the correct disk - you can and will lose your data if you format the wrong drive!!!

3) Unmount the disk

#diskutil unmountDisk disk4
Unmount of all volumes on disk4 was successful


4)  Format the drive 

#sudo newfs_msdos -F 16 /dev/disk4


After a few moments, your drive will format.

5)  Remount the drive (optional)

#diskutil mount /dev/disk4


It will probably show up as "NO NAME" on your Desktop.  You can rename it here or to do it at the time of formatting use -v Type in a Volume Name when you format the drive.