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?

@kristatos This worked great. Thank you.
(I did have to delete the cache, then restart to rebuild it.)

I'm using a MicroSD card for extra storage on my Sony Xperia X. No matter where I place audio files on the SD card, they are added to the music library. This is inconvenient as I need to store non-music mp3's on the card. Is there any way that I can restrict the Music app's access to just the Music directory?

@Jimpson ,
for me it was in ~/.local/share/ubuntu-download-manager/com.mikeasoft.podbird

but if its not there, try using the du command to search for large directories. I started from the very top at ~ and started using this:

du -d 1 --block==1G

^this command shows the size of each directory/file in ur current location in terms of gigabytes. podbird should be up there in size, so just start working ur way down