How can I determine whether a file is on a USB mass storage device?

Question

Given a File or a Uri, how can I reliably determine whether or not the file is located on a USB mass storage device connected to the phone's USB port?

My test phone, a Samsung Galaxy S7 Edge, has a USB mass storage device connected, an internal (physical) micro SD card, and internal storage. When I query Context.getExternalCacheDirs(), I see all three of them:

/storage/emulated/0/Android/data/...
/storage/A2B3-15F3/Android/data/...
/storage/3535-3038/Android/data/...

How can I determine which one is the USB mass storage device? (It's A2B3-15F3, by the way).

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Research

I looked at this answer (the StorageHelper.resolveType() method), but as you can see, the device's mounted file path does not contain the string "usb".

I then tried using the StorageManager service. With the devices above we can get a StorageVolume` using:

StorageManager storageManager = context.getSystemService(StorageManager.class);
StorageVolume volume = storageManager.getStorageVolume(context.getExternalCacheDirs()[1]);

This is where things got a little odd. The StorageVolume API docs and my local copy of the file in the Android 25 sources don't appear to be much help to me, but when I execute the above code in the Android Studio debugger the object contains an mSubSystem field that describes the three devices above as "fuse", "usb" and "sd" respectively. Here is an example:

Why do I see this information in the debugger, but not the source? Could this field be specific to the Android distribution for this particular phone?

Answer 1

Environment.getExternalStorageDirectory() will return the path of SD card mount point. USB storage is only supported on very few devices (those that support USB host mode for external storage). This will be mounted somewhere under /mnt, but the exact location will vary. You will need to rather use Android NDK to interrogate and iterate mounted devices to find the one you're after. Writing logic under NDK & query inside /mnt is a reliable route.

If you really want to try on java side, try out the following code, if it works for you:

public String getUSBStoragepath() {
    try {
        Runtime runtime = Runtime.getRuntime();
        Process proc = runtime.exec("mount");
        InputStream input_stream = proc.getInputStream();
        InputStreamReader isr = new InputStreamReader(input_stream);
        String line;
        String[] paths_list = new String[10];
        int i = 0;
        int available = 0;

        BufferedReader buffer_reader = new BufferedReader(isr);
        while ((line = buffer_reader.readLine()) != null) {
            String mount = new String();
            if (line.contains("secure"))
                continue;
            if (line.contains("asec"))
                continue;

            if (line.contains("fat")) {// TF card
                String columns[] = line.split(" ");
                if (columns != null && columns.length > 1) {
                    mount = mount.concat(columns[1] + "/requiredfiles");

                    paths_list[i] = mount;
                    i++;

                    // check directory is exist or not
                    File dir = new File(mount);
                    if (dir.exists() && dir.isDirectory()) {
                        // do something here

                        available = 1;
                        final_path = mount;
                        break;
                    } else {

                    }
                }
            }
        }
        if (available == 1) {

        } else if (available == 0) {
            final_path = paths_list[0];
        }

    } catch (Exception e) {

    }
    return final_path;
}

You will then need to query in the final path to see if the file in question exists or not to determine if the given file is in USB or not.

Answer 2

You can check if usb mass storage is connected to phone via USBManager, list of connected USB devices, then check USBInterface if it class is USB_CLASS_MASS_STORAGE. Then ask user for disconnect USB mass storage, save SD Card path and ask user for reconnecting USB. Rest of dirs will be USB path.