[X 2014 Q&A, Help & Troubleshooting] sgpieter:
My personal experience and a guide for using a USB otg as internal storage

Dear tanohack,

I ran into the same storage issue a while back and now I have a small micro usb stick configured as adoptable storage. My phone is not rooted (and you need Android 6.0 to perform this). This guide specifically covers how to transfer apps to an USB stick and how to use the stick is internal storage. If you only intend to store music or media on the stick this is not necessary.

tldr: It is indeed possible to have a USB configured as internal storage with a simple ADB command, possibly a great solution to your problem. This can, however, reduce the performance of intensive games on the stick, and it could be cumbersome to carry the stick around.

How to enable the adoption of an usb stick

It is possible to adopt an usb without root, but this is not enabled by default. This is primarily meant for developers to test their apps, but we can use it as well.

To enable it, you only have to perform the following ADB command once:

adb shell sm set-force-adoptable true

This is officially mentioned here, but I found it either on Pockednow or Android Central.

If you don't already have ADB you can follow this guide to set it up

The two storage modes and their tradeoffs

When you adopt your external storage medium (the USB in our case) you can choose to ether, only store specific apps (Use as portable storage), or to move your whole storage over to the stick (Use as internal storage). Both options have their pros and cons.

First off, the apps only version. When you use this, only apps could be moved over to the USB. System apps and apps that have disabled it can't be moved over. For all other apps, go to their storage info in the settings to manage their location. If you were to remove the USB stick the apps will disappear from your app drawer and the icons on your home screen will get grayed out.

The downside of this mode is that large games have stored a lot of their data in obb files that stay on your device, even if you move the game itself over to the USB. Your pictures, videos and music files will also stay on your device.

So, how about the second mode? In the second mode, the complete filesystem will be transferred over to the USB, even those obb files. Individual apps can still be moved between the USB and the phone (system apps etc. still stay on your phone).

Great no storage problems anymore, but what are the cons?

First of, I noticed some reduced performance mainly when loading a large game, and while playing an intensive game the frame rate could be a bit more inconsistent.

The biggest con of this mode, however, is that you don't have a file system if the USB is removed. Most apps will still work as natural but the main drawback is that you can't take pictures or videos without the USB. So if you see a cool shot, and you don't have your USB attached, you have to reattach the stick and wait for it to check and recognise it. This could take anywhere from 3 to 15 seconds.

Battery life

For some reason the phone doesn't enter deep sleep and stays awake when the screen is off while the USB is attached. This means that you can't keep the USB connected all the time or you won't be able to reach the end of the day.[/HIDE]

The USB I personally used

I use a 32GB Transcend Jetflash USB stick. It is a fairly small USB stick that can plug directly into my phone. It does however get warm while using it. As I described earlier, the performance could be better but an USB connection simply can't compete with onboard storage, so it's probably not their fault. I assume every USB storage will be this fast.

How I use it at the moment

At first I went all in and and used the USB as internal storage. When I went on holiday I converted to the Portable storage mode to regain the ability to take pictures at a moment's notice. Since then, I haven't moved back. Because only not frequently used apps are located on the USB and I use them even less often because they're on the stick, I haven't used the USB much lately.

miscellaneous

I use Storage Shortcut in combination with Custom Quick Settings to get a handy shortcut to the storage settings in the quick settings.