By Bilo

2016-02-23 09:12:55 8 Comments

How does the "Format as internal storage" feature work in Android 6.0+? See screenshot below:

Screenshot (click to enlarge)

Why is it the case that "this USB drive will only work in this device"? Is it due to a partition format? I have tried to read it in Windows but the file format is RAW; is it in EXT4 format so Windows can't read it? What will happen if I insert this card to another Android 6.0 device? Will the other device able to read it?


@Stefan 2017-02-26 20:12:36

There is an excellent post on reddit Let's clear up the confusion regarding storage in Android once and for all, including adoptable storage in Marshmallow.

But as far as I understand the sticking point is still that developers still control whether their app is movable to SD, in this case the extended internal storage. So you can still run into the limitation of the phones build in storage. At at least I can allocate all free space to the /data partition by repartition it (there are more handy .pit files for Samsung device), since I won't need any /media partition since as far as I understand, Android will move all user data, pictures, offline files of streaming services etc to the integrated SD card; please confirm or correct.

@Milind R 2018-01-22 14:44:00

There is NO /media partition : the "fake" /SDCard/ folder which holds data that you can view from a computer via USB connection with the phone is just one directory in /data, usually /data/media/0 . This folder is moved to the adopted SD card. The space freed up in /data is automatically available for other apps.

@Izzy 2016-02-23 09:39:38

If you use this feature, your external SD card is replacing your internal storage. In the process, it will be encrypted. You no longer can "simply unmount" the card to read it in any other device (including a card reader attached to your PC), as in that case the file system would be unreadable in the latter (due to encryption – which is done "for security", so a thief cannot simply remove the card to circumvent your device's lock screen and get straight to your data).

Reference: Android 6.0 can treat SD cards as internal storage… at a cost

Choose internal storage and the microSD card will be reformatted and encrypted. Once this is done, the card can only be used as internal storage. If you try to eject the card and read it on a computer, it won’t work. All data on the card will also be erased, so you may want to back up anything important first. Android does provide an option to migrate it, but early reports suggest it’s unreliable.


You can still choose to have apps installed to either the true internal storage or your microSD card that’s been formatted to behave like internal storage. But if you had a phone with 8GB of storage and a 32GB microSD card, you’ll only have 32GB of space for music, movies, games, or other files, not 40GB.

Reference: Adoptable Storage: Android Marshmallow’s hidden feature that your phone probably can’t use:

Notice that the space is not increased by the size of the sdcard, but to the size of the sdcard. You’re basically swapping out your internal storage (which is typically eMMC, which is fairly quick), with your sdcard (which is typically slower than the built-in storage chip). You’ll be trading speed for capacity. In some cases this is a fair trade.

There is one more interesting tidbit: with a little bit of work, adoptable storage extends beyond just microsd cards. You can technically mount any USB OTG device as adoptable storage. In theory, this would let you have a 1TB hard drive attached via USB OTG. Of course you’d never be able to remove that hard drive, so the uses are very limited – but very interesting.

If you’re interested in forcing “any” storage device connected via USB OTG to become adoptable storage, here’s the command you’ll need to know. We haven’t tried this, so proceed at your own risk.

   adb shell sm set-force-adoptable true

Where AndroidCentral in Inside Marshmallow: Adoptable storage continues:

   adb shell sm set-force-adoptable true

But you probably shouldn't.

Once a storage device is adopted, it becomes part of the system and is no longer removable. Sure you can physically remove it, but you'll be prompted to put it back while apps and services crash on your phone or tablet. It's adopted — taken in and loved by the system, and made part of the whole.

An interesting comment there reads:

Glad I read this, I was thinking adoptable storage was a great idea and I could buy a 16GB phone and simply throw in a big microSD card and beat the system. But, probably not a good idea now that I understand more of how it works.

Plus, to underline what I wrote about :

You can remove and reformat the card whenever you like, but doing so will force you to factory reset your phone.


I opted to keep it as normal external storage. At least that way if the phone has a failure i still have access to my SD card files. The other way keeps it encrypted and if the phone were to experience phone ending glitch the card would be useless as its encrypted and the only device to see it is dead. No taking it out and inserting into a computer to recover my pics or vids or music.

According to Android 6.0 Marshmallow tip: ‘Adopt’ your MicroSD card as internal storage [Video], there is a "way back":

Thankfully, it’s not completely permanent. If you decide you’d like to use it just to store media, you can change it back by heading to Settings>Storage & USB, then selecting the card, hitting the top right menu and then select ‘Format as portable’.

But read between the lines: Format means all data on it will be erased. So if you want to do that, you'll need to make sure having secured your data beforehand. No easy "USB card reader" way here, you'll have to copy everything via USB or network.1 And if the card breaks, no way to use the card reader and some recovery soft to rescue at least parts of it; encryption makes that impossible. If it's broken it most likely won't even mount on the device anymore – so everything you haven't backed up elsewhere will be "gone for good".

As long as the card is "good", there's some interesting pointer found in Decrypting Android M adopted storage (emphasis mine):

Android M allows for adoptable storage, which is implemented similarly to internal storage FDE -- using dm-crypt with a per-volume, static 128-bit AES key, stored in /data/misc/vold/. Once the key is extracted from the device, adopted storage can be mounted and read/written on any Linux machine. Adoptable storage encryption is done purely in software (at least in the current preview build), so its performance is likely comparable to encrypted internal storage on devices that don't support hardware-accelerated FDE.

Further readings:

1In Storage › Internal Storage you should find "migrate data" which will move your stuff back to internal – if you got enough space left there. (thanks to Barleyman for pointing this out)

@beeshyams 2016-02-23 10:12:19

Thanks. It seems a little odd that both file systems cannot be read ? What happens if ext SD gets corrupted ?

@Izzy 2016-02-23 10:30:23

@beeshyams then congrats – and pray you've got a decent backup: factory-reset, put in a new card, restore. Guess why I'd never use this "feature". And you forgot one more issue: what if your 16GB card gets filled up, and you want to replace it by a 32GB or 64GB card? Hint: the same … // Further readings: Android 6.0 can treat SD cards as internal storage… at a cost

@Izzy 2016-02-23 11:27:17

@beeshyams See my update for more details :)

@beeshyams 2016-02-23 11:48:41

+1. Educative and seems to be a cr***y idea to use it except as in earlier versions. Bookmarked this for reference

@Izzy 2016-02-23 13:04:08

Me too, @beeshyams – and feel welcome to our new adoptable-storage tag-wiki :)

@Bilo 2016-02-23 13:07:11

+1. Very detailed explanation for including the master key part. Just one more thing in curious, let say if I want to upgrade the 16GB SD to 32GB SD, if I had backup the master key, I should be able to replace it then restore the master key to make it functional without factory reset?

@Izzy 2016-02-23 13:11:16

I doubt that it works this way. It's rather: 1) backup the entire SD card (might require root for some hidden parts), then 2) format it back to external, 3) replace and optionally "adopt" it again, 4) restore. As soon as you plug in a different card, Android will either ask to adopt that again (new encryption), or refuse it altogether (as it needs the adopted one). Have not tried that (am not crazy enough to), so I cannot tell for sure which one it will be.

@Matthew Read 2016-03-15 05:27:31

So does the "real" internal storage become completely wasted, then? Or is it added to /system or something?

@Izzy 2016-03-15 07:26:06

@MatthewRead As far as I understood, rather the former. It's definitely not added to /system (or melted into any other partition), neither is it "swapped" (so it could be dealt with as "external storage"). It's even difficult to access at all from what I've read. Cannot speak from my own experience, though: Don't have MM – and if I had, I'd definitely not use adoptable storage. But that's me and my use case, it would simply make no good sense to me: more cons, no pros left (I have no "huge apps", etc.).

@William Everett 2017-01-25 22:03:25

It's worth noting that removing the sd card does not actually require a factory reset. Android is sort of smart and you basically lose access the data on the card. For apps that were entirely on the card, they just don't appear anywhere in the system anymore. For apps that were still installed on the internal storage (system and some user installed apps) and had data stored on the adopted storage, you'll get error messages and app crashes.

@Milind R 2018-01-22 14:39:55

@Izzy: My experience directly contradicts your assertion that the "real" internal storage becomes "wasted" when an external SD card is adopted. On my phone it shows 15.69 GB available (14.96 GB used of 30.66 GB). 14.69 GB = 6.85 GB (internal) + 2.80 GB (adopted). It also doesn't square with what I learned from this comprehensive Reddit post on adoptable storage.

@Izzy 2018-01-22 17:31:12

@MilindR I wrote this up when the feature became available with MM about 2 years ago (and that Reddit was written about 1 year ago). Things might be different with later updates/Android versions – and from feedback I've received within the last few weeks, it certainly seems they are. Be welcome to write up a more up-to-date answer and ping me back then to update mine; as I'm not using adoptable storage myself (and don't plan to), I don't feel qualified enough :)

@Barleyman 2016-05-24 15:09:16

A second answer to have some points more easily accessible.

  1. Yes the internal storage is still accessible, to android at least. After you've done "migrate data" to external storage, everything moveable goes to sdcard but some things still remain on internal. On galaxy S5 you can find these in /data/app/
  2. You can reverse the process by opening "internal storage" from storage settings and there's "migrate data" that will copy everything back to internal nvram.
  3. If (when) you don't have enough space to move everything, you can still do it the hard way by going to app info and using the change (location) button.

Titanium backup does restore everything to internal storage. Since in Galaxy S5 at least the reported available storage is wrong, it'll fill it right up. At that point you start getting constant popups from crashing apps and you have to work your way to appinfo for some large app and either uninstall it or move it to external to regain control.

There is a way of batch moving apps between internal and external, apps2sd all in one. It's a bit slow to process the files but it's a hands off process.

@Michael 2020-01-04 23:09:31

Ok, here's what I want to know. How to I start using the external SD card instead of the internal storage without moving anything? I understand my internal data won't be available until I detach adopted storage, and I'm fine with that, but the adopted storage is smaller than my internal storage (even just the stuff on internal storage) so I cannot move anything. What I see is that Android has added 2x the space of the external device to total available, but then claims it's 100% used by "System", making none of it actually available!

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