How to split your SD card between adopted internal storage and portable external storage.(see how-to instructions below)
Fed up of waiting on Motorola for the Marshmallow upgrade on my XT1072, and being short of internal storage, and being very fed up of having to move-to-sd apps after upgrade (along with occasional tricks such as deleting all data for drive, google+, chrome to scavenge extra memory when clear-cache wasn't enough) I decided to upgrade to CyanogenMod 13 to get Marshmallow that way.
I was really looking forward to being able to use my SD card as internal memory and have no more problems about storage.
What really happens is that with Marshmallow you cannot move-to-sd without adopting your SD card as internal. And then the only apps that will move-to-sd (now called Change) are those that could already move-to-sd.
And one of those apps that won't move-to-sd (even after the adoption of the SD card as internal storage) is Google Music.
Only now there is no SD card for Google Music to store the music on. So it stores it on the *internal* memory.
Whaaaat! My music collection is way bigger than my app collection, how does storing my music internally instead of some apps (all of which could move to SD anyway) help anything?
After messing abuot with Links2SD, Apps2SD, root shells and mount points, reading about volume manager and changing API's I decided that even loopback file fat32 systems in the adopted storage probably wouldn't work.
So I looked at re-partitioning an adopted storage card to shrink the adopted partition to make room for a fat32 partition.
In searching how to access the encrypted partition outside of Android (so that I could resize the file system within it) I came across these notes http://nelenkov.blogspot.co.uk/2015/06/decrypting-android-m-adopted-storage.html I came across the sm command which can create a mixed or public or private volume.
Mixed turned out to be exactly what I was looking for!
How to split your card
I found how to partition my SD card to give 8G as internal storage to which all apps that can be moved will be moved and leave ~20GB as portable storage to hold music, etc.
First, you need adb working, and your SD card inserted and formatted as portable.
$ adb shell sm list-disks adoptable disk:179_64
disk:168_64 is my SD card that can be made adoptable, I want 75% as external SD:
$ adb shell sm partition disk:179_64 mixed 75
After erasing the entire SD card, it gives me 75% as portable storage and the rest as adopted internal storage.
However I advise a reboot after setting the new music storage location this as Google Music may get the wrong idea about much space is available.
Also try combing that with:
$ adb shell pm set-install-location 2
to have apps installed on the storage by default where possible. It is very effective. (Location 1 means internal, and 0 means auto-choose, but I don't know on what criteria).