assets通常存储音频视频文件，但不要太大。
Environment可以获取sd卡的相关信息，

sd卡的根路径：/storage/sdcard

package com.sxt.day06_07;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import android.widget.Toast;

public class MainActivity extends Activity {

    private static final String FILE_NAME = "sxt_logo.png";
//将工程assets下的sxt_logo.png图片保存进sd卡
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if(Environment.getExternalStorageState().equals(Environment.MEDIA_UNMOUNTED)){//MEDIA_UNMOUNTED没有挂载
            //若没有sd卡
            Toast.makeText(this, "sd卡没有准备好", 2000).show();
            return ;
        }
        InputStream in = null;
        OutputStream out=null;
        try {
            in=getAssets().open(FILE_NAME);//getAssets返回AssetManager对象(管理Asset的管理器)，获取了工程磁盘assets下的sxt_logo.png图片
            File dir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES);//获取sd卡的保存图pain的路径/storage/sdcard/Pictures
            File file=new File(dir, FILE_NAME);
            out=new FileOutputStream(file);
            int len;
            byte[] buffer=new byte[8*1024];//每次读取8K
            while((len=in.read(buffer))!=-1){
                out.write(buffer, 0, len);//将内存中的buffer写到输出流代表的sd卡
            }
            Toast.makeText(MainActivity.this, "文件保存完毕", 2000).show();
        } catch (IOException e) {
            e.printStackTrace();
        }finally{
            try {
                if(out!=null){
                    out.close();
                }
                if(in!=null){
                    in.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.sxt.day06_07"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="18" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>   申请读sd卡的权限
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>         申请写sd卡的权限
    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.sxt.day06_07.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>