Universal way to write to external SD card on Android

java android android-sdcard android-external-storage

Summary

You can grant read/write access to external SD card on the different api levels (API23+ at run time).

Since KitKat, permissions are not necessary if you use app-specific directories, required otherwise.

Universal way:

The history says that there is no universal way to write to external SD card but continues...

This fact is demonstrated by these examples of external storage configurations for devices.

API-based way:

Prior to KitKat try to use Doomsknight method 1, method 2 otherwise.

Request permissions in manifest (Api < 23) and at run time (Api >= 23).

Recommended way:

ContextCompat.getExternalFilesDirs solves the access error when you don't need to share files.

The secure way of sharing it is to use a content provider or the new Storage Access Framework.

Privacy-aware way:

As of Android Q Beta 4, apps that target Android 9 (API level 28) or lower see no change, by default.

Apps targeting Android Q by default (or opting into it) are given a filtered view into external storage.

  1. Initial answer.

Universal way to write to external SD card on Android

There is no universal way to write to external SD card on Android due to continuous changes:

  • Pre-KitKat: official Android platform has not supported SD cards at all except for exceptions.

  • KitKat: introduced APIs that let apps access files in app-specific directories on SD cards.

  • Lollipop: added APIs to allow apps to request access to folders owned by other providers.

  • Nougat: provided a simplified API to access common external storage directories.

  • ... Android Q privacy change: App-scoped and media-scoped storage

What is the better way to grant read/write access to external SD card on different API levels

Based on Doomsknight's answer and mine, and Dave Smith and Mark Murphy blog posts: 1, 2, 3:

  • Ideally, use the Storage Access Framework and DocumentFile as Jared Rummler pointed. Or:
  • Use your app specific path/storage/extSdCard/Android/data/com.myapp.example/files.
  • Add read/write permission to manifest for pre-KitKat, no permission required later for this path.
  • Try to use your App path and Doomsknight's methods considering KitKat and Samsung case.
  • Filter and use getStorageDirectories, your App path and read/write permissions prior to KitKat.
  • ContextCompat.getExternalFilesDirs since KitKat. Considering devices that return internal first.
  1. Updated answer.

Update 1. I tried Method 1 from Doomknight's answer, with no avail:

As you can see I'm checking for permissions at runtime before attempting to write on SD...

I would use application-specific directories to avoid the issue of your updated question and ContextCompat.getExternalFilesDirs() using getExternalFilesDir documentation as reference.

Improve the heuristics to determine what represents removable media based on the different api levels like android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.KITKAT

... But I get an access error, tried on two different devices: HTC10 and Shield K1.

Remember that Android 6.0 supports portable storage devices and third-party apps must go through the Storage Access Framework. Your devices HTC10 and Shield K1 are probably API 23.

Your log shows a permission denied exception accessing /mnt/media_rw, like this fix for API 19+:

<permission name="android.permission.WRITE_EXTERNAL_STORAGE" >
<group gid="sdcard_r" />
<group gid="sdcard_rw" />
<group gid="media_rw" /> // this line is added via root in the link to fix it.
</permission>

I never tried it so I can not share code but I would avoid the for trying to write on all the returned directories and look for the best available storage directory to write into based on remaining space.

Perhaps Gizm0's alternative to your getStorageDirectories() method it's a good starting point.

ContextCompat.getExternalFilesDirs solves the issue if you don't need access to other folders.

  1. Android 1.0 .. Pre-KitKat.

Prior to KitKat try to use Doomsknight method 1 or read this response by Gnathonic.

public static HashSet<String> getExternalMounts() {
    final HashSet<String> out = new HashSet<String>();
    String reg = "(?i).*vold.*(vfat|ntfs|exfat|fat32|ext3|ext4).*rw.*";
    String s = "";
    try {
        final Process process = new ProcessBuilder().command("mount")
                .redirectErrorStream(true).start();
        process.waitFor();
        final InputStream is = process.getInputStream();
        final byte[] buffer = new byte[1024];
        while (is.read(buffer) != -1) {
            s = s + new String(buffer);
        }
        is.close();
    } catch (final Exception e) {
        e.printStackTrace();
    }

    // parse output
    final String[] lines = s.split("\n");
    for (String line : lines) {
        if (!line.toLowerCase(Locale.US).contains("asec")) {
            if (line.matches(reg)) {
                String[] parts = line.split(" ");
                for (String part : parts) {
                    if (part.startsWith("/"))
                        if (!part.toLowerCase(Locale.US).contains("vold"))
                            out.add(part);
                }
            }
        }
    }
    return out;
}

Add the next code to your AndroidManifest.xml and read Getting access to external storage

Access to external storage is protected by various Android permissions.

Starting in Android 1.0, write access is protected with the WRITE_EXTERNAL_STORAGE permission.

Starting in Android 4.1, read access is protected with the READ_EXTERNAL_STORAGE permission.

In order to ... write files on the external storage, your app must acquire ... system permissions:

<manifest ...>
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
</manifest>

If you need to both..., you need to request only the WRITE_EXTERNAL_STORAGE permission.

Read Mark Murphy's explanation and recommended Dianne Hackborn and Dave Smith posts

  • Until Android 4.4, there was no official support for removable media in Android, Starting in KitKat, the concept of “primary” and “secondary” external storage emerges in the FMW API.
  • Prior apps are just relying on MediaStore indexing, ship with the hardware or examine mount points and apply some heuristics to determine what represents removable media.
  1. Android 4.4 KitKat introduces the Storage Access Framework (SAF).

Ignore the next note due to bugs, but try to use ContextCompat.getExternalFilesDirs():

  • Since Android 4.2, there has been a request from Google for device manufacturers to lock down removable media for security (multi-user support) and new tests were added in 4.4.
  • Since KitKat getExternalFilesDirs() and other methods were added to return a usable path on all available storage volumes (The first item returned is the primary volume).
  • The table below indicates what a developer might try to do and how KitKat will respond: enter image description here

Note: Beginning with Android 4.4, these permissions are not required if you're reading or writing only files that are private to your app. For more info..., see saving files that are app-private.

<manifest ...>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"
                     android:maxSdkVersion="18" />
</manifest>

Also read Paolo Rovelli's explanation and try to use Jeff Sharkey's solution since KitKat:

In KitKat there's now a public API for interacting with these secondary shared storage devices.

The new Context.getExternalFilesDirs() and Context.getExternalCacheDirs() methods can return multiple paths, including both primary and secondary devices.

You can then iterate over them and check Environment.getStorageState() and File.getFreeSpace() to determine the best place to store your files.

These methods are also available on ContextCompat in the support-v4 library.

Starting in Android 4.4, the owner, group and modes of files on external storage devices are now synthesized based on directory structure. This enables apps to manage their package-specific directories on external storage without requiring they hold the broad WRITE_EXTERNAL_STORAGE permission. For example, the app with package name com.example.foo can now freely access Android/data/com.example.foo/ on external storage devices with no permissions. These synthesized permissions are accomplished by wrapping raw storage devices in a FUSE daemon.

With KitKat your chances for a "complete solution" without rooting are pretty much zero:

The Android project has definitely screwed up here. No apps get full access to external SD cards:

  • file managers: you cannot use them to manage your external SD card. In most areas, they can only read but not write.
  • media apps: you cannot retag/re-organize your media collection any longer, as those apps cannot write to it.
  • office apps: pretty much the same

The only place 3rd party apps are allowed to write on your external card are "their own directories" (i.e. /sdcard/Android/data/<package_name_of_the_app>).

The only ways to really fix that require either the manufacturer (some of them fixed it, e.g. Huawei with their Kitkat update for the P6) – or root... (Izzy's explanation continues here)

  1. Android 5.0 introduced changes and the DocumentFile helper class.

getStorageState Added in API 19, deprecated in API 21, use getExternalStorageState(File)

Here's a great tutorial for interacting with the Storage Access Framework in KitKat.

Interacting with the new APIs in Lollipop is very similar (Jeff Sharkey's explanation).

  1. Android 6.0 Marshmallow introduces a new runtime permissions model.

Request permissions at runtime if API level 23+ and read Requesting Permissions at Run Time

Beginning in Android 6.0 (API level 23), users grant permissions to apps while the app is running, not when they install the app ... or update the app ... user can revoke the permissions.

// Assume thisActivity is the current activity
int permissionCheck = ContextCompat.checkSelfPermission(thisActivity,
        Manifest.permission.WRITE_EXTERNAL_STORAGE);

Android 6.0 introduces a new runtime permissions model where apps request capabilities when needed at runtime. Because the new model includes the READ/WRITE_EXTERNAL_STORAGE permissions, the platform needs to dynamically grant storage access without killing or restarting already-running apps. It does this by maintaining three distinct views of all mounted storage devices:

  • /mnt/runtime/default is shown to apps with no special storage permissions...
  • /mnt/runtime/read is shown to apps with READ_EXTERNAL_STORAGE
  • /mnt/runtime/write is shown to apps with WRITE_EXTERNAL_STORAGE
  1. Android 7.0 provides a simplified API to access external storage dirs.

Scoped Directory Access In Android 7.0, apps can use new APIs to request access to specific external storage directories, including directories on removable media such as SD cards...

For more information, see the Scoped Directory Access training.

Read Mark Murphy posts: Be Careful with Scoped Directory Access. It was deprecated in Android Q:

Note that the scoped directory access added in 7.0 is deprecated in Android Q.

Specifically, the createAccessIntent() method on StorageVolume is deprecated.

They added a createOpenDocumentTreeIntent() that can be used as an alternative.

  1. Android 8.0 Oreo .. Android Q Beta changes.

Starting in Android O, the Storage Access Framework allows custom documents providers to create seekable file descriptors for files residing in a remote data source...

Permissions, prior to Android O, if an app requested a permission at runtime and the permission was granted, the system also incorrectly granted the app the rest of the permissions that belonged to the same permission group, and that were registered in the manifest.

For apps targeting Android O, this behavior has been corrected. The app is granted only the permissions it has explicitly requested. However, once the user grants a permission to the app, all subsequent requests for permissions in that permission group are automatically granted.

For example, READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE...

Update: An Android Q earlier beta release temporarily replaced the READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE permissions with more fine-grained, media-specific permissions.

Note: Google introduced roles on Beta 1 and removed them from the documentation before Beta 2...

Note: The permissions specific to media collections that were introduced in earlier beta releases—READ_MEDIA_IMAGES, READ_MEDIA_AUDIO, and READ_MEDIA_VIDEO—are now obsolete. More info:

Q Beta 4 (final APIs) review by Mark Murphy: The Death of External Storage: The End of the Saga(?)

"Death is more universal than life. Everyone dies, but not everyone lives." ― Andrew Sachs

  1. Related questions and recommended answers.

How can I get external SD card path for Android 4.0+?

mkdir() works while inside internal flash storage, but not SD card?

Diff between getExternalFilesDir and getExternalStorageDirectory()

Why getExternalFilesDirs() doesn't work on some devices?

How to use the new SD card access API presented for Android 5.0 (Lollipop)

Writing to external SD card in Android 5.0 and above

Android SD Card Write Permission using SAF (Storage Access Framework)

SAFFAQ: The Storage Access Framework FAQ

  1. Related bugs and issues.

Bug: On Android 6, when using getExternalFilesDirs, it won't let you create new files in its results

Writing to directory returned by getExternalCacheDir() on Lollipop fails without write permission


I believe there are two methods to achieve this:

METHOD 1: (does NOT work on 6.0 and above, due to permission changes)

I have been using this method for years on many device version with no issue. Credit is due to the original source, as it was not me who wrote it.

It will return all mounted media (including Real SD Cards) in a list of strings directory locations. With the list you can then ask the user where to save, etc.

You can call it with the following:

 HashSet<String> extDirs = getStorageDirectories();

Method:

/**
 * Returns all the possible SDCard directories
 */
public static HashSet<String> getStorageDirectories() {
    final HashSet<String> out = new HashSet<String>();
    String reg = "(?i).*vold.*(vfat|ntfs|exfat|fat32|ext3|ext4).*rw.*";
    String s = "";
    try {
        final Process process = new ProcessBuilder().command("mount")
                .redirectErrorStream(true).start();
        process.waitFor();
        final InputStream is = process.getInputStream();
        final byte[] buffer = new byte[1024];
        while (is.read(buffer) != -1) {
            s = s + new String(buffer);
        }
        is.close();
    } catch (final Exception e) {
        e.printStackTrace();
    }

    // parse output
    final String[] lines = s.split("\n");
    for (String line : lines) {
        if (!line.toLowerCase().contains("asec")) {
            if (line.matches(reg)) {
                String[] parts = line.split(" ");
                for (String part : parts) {
                    if (part.startsWith("/"))
                        if (!part.toLowerCase().contains("vold"))
                            out.add(part);
                }
            }
        }
    }
    return out;
}

METHOD 2:

Use the v4 support library

import android.support.v4.content.ContextCompat;

Just call the following to get a list of File locations of storage.

 File[] list = ContextCompat.getExternalFilesDirs(myContext, null);

The locations differ in usage however though.

Returns absolute paths to application-specific directories on all external storage devices where the application can place persistent files it owns. These files are internal to the application, and not typically visible to the user as media.

External storage devices returned here are considered a permanent part of the device, including both emulated external storage and physical media slots, such as SD cards in a battery compartment. The returned paths do not include transient devices, such as USB flash drives.

An application may store data on any or all of the returned devices. For example, an app may choose to store large files on the device with the most available space

More Info on ContextCompat

They are like app specific files. Hidden from other apps.


Just another answer. This answer only shows 5.0+ because I believe Doomknight's answer posted here is the best way to do for Android 4.4 and below.

This is originally posted here (Is there a way to get SD Card size in Android?) by me to get the external SD Card's size on Android 5.0+

To get the External SD card as a File:

public File getExternalSdCard() {
    File externalStorage = null;
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
        File storage = new File("/storage");

        if(storage.exists()) {
            File[] files = storage.listFiles();

            for (File file : files) {
                if (file.exists()) {
                    try {
                        if (Environment.isExternalStorageRemovable(file)) {
                            externalStorage = file;
                            break;
                        }
                    } catch (Exception e) {
                        Log.e("TAG", e.toString());
                    }
                }
            }
        }
    } else {
        // do one of many old methods
        // I believe Doomsknight's method is the best option here
    }

    return externalStorage;
}

Note: I only get the "first" external sd card however you can modify it and return ArrayList<File> instead of File and let the loop continue instead of calling break after the first one is found.


In addition to all other nice answers, I could add a bit more to this question so it can give wider coverage for readers. In my answer here, I would use 2 countable resources to present External Storage.

The first resource is from Android Programming, The Big Nerd Ranch Guide 2nd edition, chapter 16, page 294.

The book describes the basic and external file and directory methods. I will try to make a resume of what could be relevant to your question.

The following part from the book:

External Storage

Your photo needs more than a place on the screen. Full-size pictures are too large to stick inside a SQLite database, much less an Intent. They will need a place to live on your device’s filesystem. Normally, you would put them in your private storage. Recall that you used your private storage to save your SQLite database. With methods like Context.getFileStreamPath(String) and Context.getFilesDir(), you can do the same thing with regular files, too (which will live in a subfolder adjacent to the databases subfolder your SQLite database lives in)

Basic file and directory methods in Context

| Method                                                                                |
|---------------------------------------------------------------------------------------|
|File getFilesDir()                                                                      |
| - Returns a handle to the directory for private application files.                    |
|                                                                                       |
|FileInputStream openFileInput(String name)                                             |
| - Opens an existing file for input (relative to the files directory).                 |
|                                                                                       |
|FileOutputStream openFileOutput(String name, int mode)                                 |
| - Opens a file for output, possibly creating it (relative to the files directory).    |
|                                                                                       |
|File getDir(String name, int mode)                                                     |
| - Gets (and possibly creates) a subdirectory within the files directory.              |
|                                                                                       |
|String[] fileList()                                                                    |
| - Gets a list of file names in the main files directory, such as for use with         |
|   openFileInput(String).                                                              |
|                                                                                       |
|File getCacheDir()                                                                     |
| - Returns a handle to a directory you can use specifically for storing cache files.   |
|   You should take care to keep this directory tidy and use as little space as possible|

If you are storing files that only your current application needs to use, these methods are exactly what you need.

On the other hand, if you need another application to write to those files, you are out of luck: while there is a Context.MODE_WORLD_READABLE flag you can pass in to openFileOutput(String, int), it is deprecated, and not completely reliable in its effects on newer devices. If you are storing files to share with other apps or receiving files from other apps (files like stored pictures), you need to store them on external storage instead.

There are two kinds of external storage: primary, and everything else. All Android devices have at least one location for external storage: the primary location, which is located in the folder returned by Environment.getExternalStorageDirectory(). This may be an SD card, but nowadays it is more commonly integrated into the device itself. Some devices may have additional external storage. That would fall under “everything else.”

Context provides quite a few methods for getting at external storage, too. These methods provide easy ways to get at your primary external storage, and kinda-sorta-easy ways to get at everything else. All of these methods store files in publicly available places, too, so be careful with them.

External file and directory methods in Context

| Method                                                                                |
| --------------------------------------------------------------------------------------|
|File getExternalCacheDir()                                                             |
| - Returns a handle to a cache folder in primary external storage. Treat it like you do|
|   getCacheDir(), except a little more carefully. Android is even less likely to clean |
|   up this folder than the private storage one.                                        |
|                                                                                       |
|File[] getExternalCacheDirs()                                                          |
| - Returns cache folders for multiple external storage locations.                      |
|                                                                                       |
|File getExternalFilesDir(String)                                                       |
| - Returns a handle to a folder on primary external storage in which to store regular  |
|   files. If you pass in a type String, you can access a specific subfolder dedicated  |
|   to a particular type of content. Type constants are defined in Environment, where   |
|   they are prefixed with DIRECTORY_.                                                  |
|   For example, pictures go in Environment.DIRECTORY_PICTURES.                         |
|                                                                                       |
|File[] getExternalFilesDirs(String)                                                    |
| - Same as getExternalFilesDir(String), but returns all possible file folders for the  |
|   given type.                                                                         |
|                                                                                       |
|File[] getExternalMediaDirs()                                                          |
| - Returns handles to all the external folders Android makes available for storing     |
|   media – pictures, movies, and music. What makes this different from calling         |
|   getExternalFilesDir(Environment.DIRECTORY_PICTURES) is that the media scanner       |
|   automatically scans this folder. The media scanner makes files available to         |
|   applications that play music, or browse movies and photos, so anything that you     |
|   put in a folder returned by getExternalMediaDirs() will automatically appear in     |
|   those apps.                                                                         |

Technically, the external folders provided above may not be available, since some devices use a removable SD card for external storage. In practice this is rarely an issue, because almost all modern devices have nonremovable internal storage for their “external” storage. So it is not worth going to extreme lengths to account for it. But we do recommended including simple code to guard against the possibility, which you will do in a moment.

External storage permission

In general, you need a permission to write or read from external storage. Permissions are well-known string values you put in your manifest using the <uses-permission> tag. They tell Android that you want to do something that Android wants you to ask permission for.

Here, Android expects you to ask permission because it wants to enforce some accountability. You tell Android that you need to access external storage, and Android will then tell the user that this is one of the things your application does when they try to install it. That way, nobody is surprised when you start saving things to their SD card.

In Android 4.4, KitKat, they loosened this restriction. Since Context.getExternalFilesDir(String) returns a folder that is specific to your app, it makes sense that you would want to be able to read and write files that live there. So on Android 4.4 (API 19) and up, you do not need this permission for this folder. (But you still need it for other kinds of external storage.)

Add a line to your manifest that requests the permission to read external storage, but only up to API Listing 16.5 Requesting external storage permission (AndroidManifest.xml)

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
         package="com.bignerdranch.android.criminalintent" >
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"
         android:maxSdkVersion="18" />

The maxSdkVersion attribute makes it so that your app only asks for this permission on versions of Android that are older than API 19, Android KitKat. Note that you are only asking to read external storage. There is also a WRITE_EXTERNAL_STORAGE permission, but you do not need it. You will not be writing anything to external storage: The camera app will do that for you

The second resource is this link read all of it, but you can also jump to Using the External Storage section.

Reference:

  • Android Storage Options: https://developer.android.com/guide/topics/data/data-storage.html
  • Book site: https://www.bignerdranch.com/we-write/android-programming

More reading stuff:

  • https://developer.android.com/reference/android/os/Environment.html#getExternalStoragePublicDirectory%28java.lang.String%29
  • https://developer.android.com/reference/android/os/Environment.html#DIRECTORY_PICTURES
  • https://developer.android.com/reference/android/os/Environment.html#DIRECTORY_MOVIES
  • How can I get external SD card path for Android 4.0+?

Disclaimer: This information was taken from Android Programming: The Big Nerd Ranch Guide with permission from the authors. For more information on this book or to purchase a copy, please visit bignerdranch.com.

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