The "encrypt external sd card" option doesn't show up under Settings->Security on my LG Optimus F6, or my Huawei Y301-A2. A microSD card, FAT32 formatted and empty, is inserted and recognized correctly. The "encrypt device" option shows up, but the "encrypt external sd card" does not. How do I get this option to show up? This is using Android 4.1.2
I don't know about those exact models, but on Jelly Bean in general, you must enable a password on your phone to encrypt external SD.
Security -> Set up screen lock -> Password.
Then you'll need to set a password for the SD.
Step-by-step tutorial with images: http://visihow.com/Encrypt_Files_on_Your_SD_Card_Using_Your_Android_Phone