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<p>Android has always supported external storage accessories (such as SD cards), but
these accessories were historically limited to simple file storage, due to
their expected impermanence and the minimal data protection offered to
<a href="/devices/storage/traditional">traditional external storage</a>.
Android 6.0 introduced the ability to
<a href="http://developer.android.com/about/versions/marshmallow/android-6.0.html#adoptable-storage"
  class="external">adopt</a> external storage media to act like internal
storage.</p>

<aside class="caution">
  <p><strong>Caution</strong>: On devices running Android 7.0-8.1,
  <a href="/security/encryption/file-based">file-based encryption</a>
  (FBE) can't be used together with adoptable storage. On devices
  using FBE, new storage media (such as an SD card) must be used as
  <a href="/devices/storage/traditional">traditional storage</a>.</p>
  <p>Devices running Android 9 and higher can use adoptable storage and FBE.</p>
</aside>

<p>When external storage media is adopted, it’s formatted and encrypted to only
work with a single Android device at a time. Because the media is strongly tied
to the Android device that adopted it, it can safely store both apps and
private data for all users.</p>

<p>When users insert new storage media (such as an SD card) in an adoptable
location, Android asks them how they want to use the media. They can choose to
adopt the media, which formats and encrypts it, or they can continue using it
as-is for simple file storage. If they choose to adopt, the platform offers to
migrate the primary shared storage contents (typically mounted at
<code>/sdcard</code>) to the newly adopted media, freeing up valuable space on
internal storage. Unlike traditional storage, which is limited to 2TB due to its
use of
<a href="https://en.wikipedia.org/wiki/Master_boot_record" class="external">MBR</a>,
adoptable storage uses
<a href="https://en.wikipedia.org/wiki/GUID_Partition_Table" class="external">GPT</a>
and therefore has file storage limit of ~9ZB.</p>

<p>Apps can be placed on adopted storage media only when the developer has
indicated support through the <code>android:installLocation</code> attribute.
New installs of supported apps are automatically placed on the
storage device with the most free space, and users can move supported apps
between storage devices in the <em>Settings</em> app. Apps moved to adopted
media are remembered while the media is ejected,
and return when the media is reinserted.</p>

<h2 id="security">Security</h2>


<p>The platform randomly generates an encryption key for each adopted device,
and that key is stored on the internal storage of the Android device. This
effectively makes the adopted media as secure as internal storage. Keys are
associated with adopted devices based on the adopted partition GUID. The
adopted device is encrypted using <code>dm-crypt</code> configured with the
<code>aes-cbc-essiv:sha256</code> algorithm
and a 128-bit key size.
</p>
<p>The on-disk layout of the adopted device closely mirrors the internal data
partition, including SELinux labels, etc. When multi-user is supported on the
Android device, the adopted storage device also supports multi-user with the
same level of isolation as internal storage.</p>

<p>Because the contents of an adopted storage device are strongly tied to the
Android device that adopted it, the encryption keys should not be extractable
from the parent device, and therefore the storage device can't be mounted
elsewhere.</p>

<p>
The default encryption algorithm for contents mode is <code>aes-256-xts</code>
and for filenames is <code>aes-256-heh</code>. You can change these by changing
the values of the properties <code>ro.crypto.volume.contents_mode</code> and
<code>ro.crypto.volume.filenames_mode</code> respectively, by setting
<code>PRODUCT_PROPERTY_OVERRIDES</code> in <code>device.mk</code>.
</p>
<p>
If your kernel does not support HEH filename encryption, you can use CTS mode
instead by adding the following to device.mk:
</p>


<pre
class="prettyprint">PRODUCT_PROPERTY_OVERRIDES += \
ro.crypto.volume.filenames_mode=aes-256-cts</pre>

<h2 id="performance_and_stability">Performance and stability</h2>


<p>Only external storage media in stable locations, such as a slot inside a
battery compartment or behind a protective cover, should be considered for
adoption to help avoid accidental data loss or corruption. In particular, USB
devices connected to a phone or tablet should never be considered for adoption.
One common exception would be an external USB drive connected to a TV-style
device, because the entire TV is typically installed in a stable location.</p>

<p>When a user adopts a new storage device, the platform runs a benchmark and
compares its performance against internal storage. If the adopted device is
significantly slower than internal storage, the platform warns the user about a
possibly degraded experience. This benchmark was derived from the actual I/O
behavior of popular Android apps. Currently, the AOSP implementation will only
warn users beyond a single threshold, but device manufacturers may adapt this
further, such as rejecting adoption completely if the card is extremely slow.</p>

<p>Adopted devices must be formatted with a filesystem that supports POSIX
permissions and extended attributes, such as <code>ext4</code> or
<code>f2fs</code>. For optimal performance, the <code>f2fs</code> filesystem is
recommended for flash-based storage devices.</p>

<p>When performing periodic idle maintenance, the platform issues
<code>FI_TRIM</code> to adopted media just like it does for internal storage.
The current SD card specification does not support the <code>DISCARD</code>
command; but the kernel instead falls back to the <code>ERASE</code> command,
which SD card firmware may choose to use for optimization purposes.</p>

<h3 id="fixing-double-encryption">Fixing double encryption</h3>

<p>In Android 8.x and lower, adoptable storage didn't work with FBE. All
existing devices that have adoptable storage used
<a href="/security/encryption/full-disk">full-disk encryption</a> (FDE).
In Android 9, adoptable storage works with FBE. However, by
default, file contents are double-encrypted because adoptable storage has an
FDE and FBE layer. By default, both layers encrypt file contents, which can
slow device performance. To fix the problem of double encryption and speed up
device performance:
</p>
<ol>
  <li>Add <a href="https://android-review.googlesource.com/q/REQ_NOENCRYPT"
             class="external">these patches</a> to your kernel.</li>
  <li>To communicate this change with <code>vold</code>, add the following to
    <code>device.mk</code>:
    <pre class="prettyprint">PRODUCT_PROPERTY_OVERRIDES += ro.crypto.allow_encrypt_override=true</pre></li>
</ol>
<p>
If you set this, but the kernel patches aren't present, adoptable storage won't
work, and the <code>vold</code> logs will contain an error that it was unable
to create the dm device.
</p>
<aside class="caution">
  <strong>Caution:</strong> Don't change this flag with an OTA update because
  it changes the on-disk format of adoptable storage.
</aside>

<h2 id="testing">Testing</h2>
<p>
To test that adoptable storage is working, run this CTS test:
</p>


<pre
class="devsite-terminal devsite-click-to-copy">cts-tradefed run commandAndExit cts-dev \
    -m CtsAppSecurityHostTestCases \
    -t android.appsecurity.cts.AdoptableHostTest</pre>
<p>
To verify behavior of USB drives and SD cards when a device doesn't have a
built-in slot or when the USB connector is being used for an active adb
connection, use:
</p>
<pre class="prettyprint">
adb shell sm set-virtual-disk true
</pre>

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