ExternalStorage

Each Android-perfect gadget bolsters a common "outer stockpiling or External Storage" that you can use to spare records. This can be a removable stockpiling media(Storage media), (for example, a SD card) or an inward (non-removable) capacity. Records spared to the outer stockpiling are world-intelligible and can be changed by the client when they empower USB mass stockpiling to exchange documents(transfer files) on a PC.

Access to external storage  i.e Read or Write files.
In Android manifest.xml we have to include the code.
<manifest>
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<uses-permission android : name ="android.permission.WRITE_EXTERNAL_STORAGE/>
<uses-permission android : name ="android.permission.READ_EXTERNAL_STORAGE/>
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</manifest>

In MainActivity.java we have to write code for external storage below shown how it is,

                          //WRITE FILE //
public boolean isExternalStorageWritable() {
    String externalstate = Environment.getExternalStorageState();
    if (Environment.MEDIA_MOUNTED.equals(externalstate)) {
        return true;
    }
    return false;
}

                         //READ FILE //
public boolean isExternalStorageReadable() {
    String externalstate = Environment.getExternalStorageState();
    if (Environment.MEDIA_MOUNTED.equals(externalstate) ||
        Environment.MEDIA_MOUNTED_READ_ONLY.equals(externalstate)) {
        return true;
    }
    return false;
}