Android 6 “Marshmallow” added a new feature: format SD cards as internal storage.
Prior versions of Android always formatted SD cards in VFAT, which doesn’t support filesystem permissions and limits how the SD card can be used. You can store data – files, music, videos, etc. – but many applications can’t be installed there due to the lack of filesystem permissions.
Android 6 can format the SD card as ext4, the same filesystem used for internal storage. This makes the SD card just like internal storage, with no restrictions. This seems like a good thing, but in practice it has limitations that make it unusable. NOTE: I tried this in Cyanogenmod 13; it might work better (or worse!) on other versions of Android 6.
I assume you already know the basic facts. If not, just Google it or read Ars Technica. Here I’ll go into some details. When Android 6 detects an SD card it lets you choose how to format it: internal (ext4), or portable (VFAT).
When I selected internal, the SD card was formatted ext4 and mounted at /mnt/expand/a474aa54-1e0a-4df6-9bc1-1e202a5167fa. This looks like a GUID generated from the SD card, will probably be different for every card.
When I selected portable, the SD card was formatted VFAT and mounted at /storage/4E3F-1CFD. This too appears to be a random ID attached to the card.
I really wanted to use internal, but several reasons prevented me:
When I first turned on internal storage, it seemed to work perfectly. Several apps moved there and everything worked fine. Then I rebooted and discovered a bug – several app icons missing from my home screen, and a system modal dialog popped up saying “System is not responding, (wait) or (close)”. I selected “wait”, everything was running fine, but the apps were simply missing from the home screen. They were still there, installed on the device, working fine, nothing lost. I dragged them back to the home screen and everything worked – until the next reboot.
Turns out, the missing apps were those stored on the SD card. The apps are still there and still work, no data is lost. But every time the device is rebooted they disappear from the home screen and you get this system error message. My hypothesis is that the SD card is being mounted too late in the boot process, so when the home screen opens and the home screen launcher (for CM13, Trebuchet) needs the icons, the apps aren’t yet available because the storage mount is still “in progress”. Just a guess… of course I can’t be sure. IF this is the root cause, then the boot process should mount the SD card as early as possible and block, waiting for SD card mount to fully complete, before proceeding. This is a serious annoying bug in Android 6 or CM 13. This alone would make the internal SD option unusable, but it got worse.
You don’t get to control what is stored on the card vs. internal. Apps themselves decide, based on the app’s code or settings in the app’s Android manifest file. Problem is, internal storage is new and most app developers haven’t thought about it much. So the behaviors you get don’t always make sense.
The camera app (neither the CM13 built-in, nor Google AOSP) didn’t store its photos or movies on external storage. Yet this seems to be one of the most obvious uses for external storage. And the app’s “storage location” setting gave me an error message when I selected external storage, refusing to take any pictures. I was forced to use internal storage for photos and movies.
Some apps install to external storage, but keep their data on internal. This is the exact opposite of how SD storage should be used! For example, Sygic, whose data can be a GB or more depending on how many states you download. If there ever was a use for this feature, this is it. Yet it didn’t work. Sygic lived on external storage but stored all its data on internal.
Some apps simply can’t find the SD card when it’s formatted as internal. For example, Solid Explorer. It could find the SD card only if you’re rooted (and I was, in CM 13) and you manually navigate to the above path where it’s mounted.
When I did get apps to put their data on the SD card, I ran into endless permissions problems. Android 6 by default assigns strict permissions to all the dirs & files created by an app. In my opinion, this has always been a solution in search of a problem, creating more hassles than it solves. I found myself constantly navigating to the various dirs and files and changing their owner, group or marking permissions to 666 or 777. This quickly gets tiring, then annoying, then infuriating.
Some apps never were able to write to the SD, even after setting up permissions. Office Suite Pro was one, though there may be others.
In summary, I gave up on internal SD storage – but I still “believe” this is the right way go, hope the Google Android team fixes the above problems. However, switching back to portable storage wasn’t easy sailing either.
Before switching back to portable (each switch re-formats the SD card), I used the Android 6 storage settings to individually move each app back to internal storage. Most of them worked after being moved, but I had to uninstall & reinstall a couple of them. Then Android 6 reformatted and remounted the card.
The home screen launcher bug never occurred – yay!
External storage was recognized by the camera app and the file manager – yay!
Apps that support external storage in Android 4 and 5 worked the same way as before – yay!
But, some apps (like Office Suite Pro) still could not read or write the SD card. On Android 4 and 5 you can fix this by editing /etc/permissions/platform.xml (more detail here). On Android 6, I went to edit that same file and found the WRITE_EXTERNAL_STORAGE permission wasn’t even there! I added it and READ_EXTERNAL_STORAGE to the Android 6 file, then rebooted.
Office Suite Pro could write to the SD card – yay!
There was only 1 app I couldn’t get to use the SD card in Android 6, no matter how I configured it (internal or portable) – that was Sygic. The new way didn’t work – Sygic put itself on external but stored all its data on internal. The old way didn’t work either. Sygic detected that its files moved to the SD, rearranged them, then hangs on its white startup screen, eventually crashes. Rebooting etc. doesn’t fix this.
In summary, Android 6 made a good sporting effort to improve SD card storage. This is a great idea made unusable by poor implementation. If the Android supported filesystem links I could fix all of the problems myself (e.g. ln -s /mnt/expand/a173822/Download /storage/emulated/0/Download). But I haven’t found a way to do that – at the command line, this fails even when rooted.