I've successfully encrypted phablet on my phone, but I was also wanting to encrypt my microSD card. I started the same process I underwent (here) in my card's directory but ran into a problem: I can't create an img file large enough to fill even most of the card. Fallocate says that the file is "too big", despite not being too big for the file system or hardware. This is a different result than if it simply didn't have enough room, as it would normally continue until all storage was filled.
But this made we ask whether I really even need to do a secondary encryption. Since the device is mounted inside of phablet, doesn't that mean that everything I write to it is encrypted?