The best way I found of doing this was to factory reset your phone with the latest update using Sony PC companion. Then as soon as the phone boots go into the google play store and make sure you have app updates switched off. This means that you keep a good 2GB of internal storage from being used up. Then if you do the adoptable storage fix it writes to both partions, but primarily the adoptive one. The reason people are running out of space is because their internal partition is already full, and the phone will tell you you have no space if the initial internal partion gets full before you do the adoptable strage fix. Some data is always installed to the internal partition so you need to make sure you have enough space on it at all times.
Just bare in mind I did all this on the day of the inital update release, and the adoptable strage partiton corrupted after about a week of use. I might have just been unlucky but just bare in mind you may lose everything on your phone if you do this.
Apologies I probably should have been a bit clearer..
I did flash a completely clean MM ROM first and didn't update anything prior to setting up adoptable storage. I had ~2GB free on the internal when I set up adoptable..
I then updated all the built in apps (disabled some of them), and then installed about half of the apps I'd like to be running (this is intended to be my travel phone), and ran out of space before I could install everything.
I use a wide variety of very popular apps and only about 20% of the space used actually went too the adoptable partition (it was being used but by very few apps) instead of the internal. (2GB went to internal, 358mb went to adoptable).
I then went down all the ones that it had installed internally, and about 3 of them did have the move option to shift them to adoptable.. but all 3 cause a kernel panic and an immediate reboot when clicked. Apps that initially DID install to adoptable can be moved freely between storage types without error. The biggest issue is that a lot of apps simply refuse to use adoptable storage.
The instructions are great and it worked perfectly.. the problem is that the implementation by Google is terrible. It either needs to extend the internal storage seamlessly (as in every app will use it - as it wont know if its internal or adopted), or it needs to completely replace the internal storage. (again invisible to the apps)
I was hoping it would rescue this phone, but its as useless now as it ever was as it still fills up just as fast.
I'm going to build the MM AOSP and see if I can figure out how to make the adoptable storage simply replace the internal data partition.. as that would solve my issues.
---------- Post added at 05:47 PM ---------- Previous post was at 05:43 PM ----------
All of you guys above!!!
to have new apps installed on adoptive storage, you need to go settings/storage/adoptive partition/ top right corner ... and select migrate (move) data
It doesn't help. (I tried it) It moved a whole 37MB after the internal partition was filled.. which didnt help at all!
Also.. the description of the feature isnt clear which way it actually moves the data! That button is available for both the internal and the adopted storage, but its unclear which way it actually moves the data when you click it on each one!