How do I recover from a botched adopted storage migration? I can't take pictures or screenshots

Question

TL;DR: I messed up. Now my phone won't store pictures because it's probably trying to write to some directory that doesn't exist. How can I fix this? What directory structure is expected? I caused it by maybe screwing up a bit when moving to Adopted Storage, but that may not be important for the answer.

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I migrated to adopted storage this morning. I wanted a mixed partitioning, but accidentally got full-card adoption (i used the GUI). As part of that I suppose I migrated the existing /storage/emulated to the adopted storage.

Because I wanted a mixed setup and I had backups of my data, I just ran sm partition ... mixed 25 and nuked the existing adopted storage.

Now I can't take any pictures. If I do a screenshot, it says it couldn't save it. If I take a picture with OpenCamera, it says the same.

I would like to avoid the hassle of factory resetting if it's just a matter of creating some directories and/or creating some symlinks. I tried setting up a symlink /media/expand/<my-adopted-storage>/user -> /storage/emulated and created DCIM (mode 1777) in /storage/emulated/0/DCIM (where it used to be). That doesn't help.

Am I on the right track and just getting the details wrong, or am I screwed?

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As you can see from the tags, I'm on CyanogenMod 13 / Android 6 / Marshmallow.

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Further investigation:

Looked where OpenCamera is looking, and it's looking at /sdcard/DCIM. /sdcard is a symlink to /storage/self/primary and that's a symlink to /mnt/user/0. Tried creating a symlink to the existing DCIM there, didn't help. Maybe it needs to point to the right place in the system to slip through the security tracks? Trying a reboot and seeing where things end up.

EDIT: It was /sdcard -> /storage/self/primary -> /mnt/user/0/primary. I created /mnt/user/0/primary -> /mnt/expand/*/user/0. No go.

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There is a secondary user on the phone. When I switch to that, I get a bunch of "Unfortunately YouTube/Gallery/foo stopped." and I can't run any of the installed apps -- then I get the same message.

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I had three SD "Cards" listed in Storage & USB: Purple (successfully adopted), Grey (first adoption, now abandoned, said "not mounted") and sdcard1. I figured that maybe it's still yearning for that unmounted (nonexistent) partition, so I clicked it and chose "Forget". Now sdcard1 is corrupt (!). Will reboot again and see what it makes of it.

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Progress! After forgetting that original adoption and rebooting, now we have /sdcard -> /storage/self/primary -> /storage/emulated/0.

/storage/emulated/0 is a FUSE system that seems to have the same size as /data, so things look pretty normal now. I took a screenshot, and /sdcard/Pictures was created.

sdcard1 is back and looks fine.

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Now there was a Migrate data command on both Internal storage and the adopted storage. When I clicked it on the adopted storage, emulated/0 was remounted as a FUSE proxy for the adopted storage and screenshots now go there instead of to internal-internal. I think this is like I want it now.

Answer 1

I believe what solved my problem was that I ran Migrate data in the newest adopted storage. Until then the system was in a state where the abandoned adopted storage was the "primary storage", but wasn't present.

I'm not sure why Migrate data on Internal storage wasn't available before I abandoned the first adopted storage. Maybe all I would have had to do was to reboot and the command would have been available.

I am going to move more of the story from the answer to the question, to create a more coherent Q&A picture.