I'm currently running on a stock, 4.4.2 XT926 unlocked and rooted. I've applied the patch that allows writing to the external SD card.
What I like to do is mount directories from the external to the internal SD card for those apps that won't write to the external SD card.
Case in point, Amazon MP3 player app. It's been updated to write to the /storage/sdcard0/Music directory. So, what I've done (and this worked pre-KK) is the following:
busybox mount -o bind /storage/sdcard1/Music /storage/sdcard0/Music
This, of course, mounts the Music directory on my external (sdcard1) SD card to the Music directory on the internal SD card (sdcard0).
I've verified that if I go to /storage/sdcard0/Music that it is indeed the Music directory on the external SD card as I can see all the music directories there.
However, if I go and download music via Amazon MP3 player, it seems to ignore the mounting and directly populates the data into the /storage/sdcard0/Music directory on the internal card. I can't see it until I unmount the directory and then I'll see them there.
Is there a way to force this to write across the mount rather than ignoring it?
The same happens with the Google Camera since it won't allow me to write to the external SD card. So, it ignores the mount I've placed on the DCIM directory and writes directly to the internal SD card.
This is annoying but I understand why some of it was implemented, but still, give me the option - don't take it away from me. Quit being like Apple! 
Any ideas?
