Try going to Settings > Storage. At the bottom you'll see storage location, switch from from phone/internal to SD card. Not entirely sure if this will fix your problem, but hope this helps

 

It seems that at this time the sd card is simply for pictures and videos both taken by the camera or uploaded by you on your pc.

 

If you can backup the app data to the phone's internal memory, then you should be able to copy it from the internal memory to the SD card using the built-in Files app. This app shows both internal memory as well SD memory, and allows you to transfer/copy/paste between the two.

I'm assuming Astro is only letting you access one on these two.

 

I can confirm 100% that you can copy all the info on the internal SD (e.g. internal storage) and paste into to the External SD (for backup). Or you could even copy and paste it onto your PC or wherever you want to keep "backups" stored.

I find this to be extremely useful because I can separate the files I added (e.g. camera pics taken, wallpapers, ringtones, notifications, etc...) from the files that I moved to the "SD" on the internal storage. At first, I thought it was bad how they partitioned 3GB for apps and 8.92GB for whatever else, but then I realized that the apps that are downloaded, when moved to the SD card, really go to the 8.92GB of space (which is what I had wanted anyway). I actually think this makes for more stability as far as the apps are concerned.