一、判断SD卡使用情况

if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))
{
    Toast.makeText(MainActivity.this,"SD卡准备就绪！",Toast.LENGTH_SHORT).show();
}
else
{
    Toast.makeText(MainActivity.this,"SD卡不存在或者未挂载！",Toast.LENGTH_SHORT).show();
}

二、获取SD卡的储存情况

File ext=Environment.getExternalStorageDirectory();
long usableSpace=ext.getUsableSpace();
long totalSpace=ext.getTotalSpace();
String usableSize= Formatter.formatFileSize(MainActivity.this,usableSpace);
String totalSize=Formatter.formatFileSize(MainActivity.this,totalSpace);
Toast.makeText(MainActivity.this,"usableSize:"+usableSize+"--"+totalSize,Toast.LENGTH_SHORT).show();

三、权限问题

//在AndroidManifest中添加SD卡读写的权限
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

//从Android6.0开始需要动态授权
Manifest.permission.READ_EXTERNAL_STORAGE
Manifest.permission.WRITE_EXTERNAL_STORAGE

四、权限申请

//动态申请权限方法
private static final int REQUEST_EXTERNAL_STORAGE = 1;
private static String[] PERMISSIONS_STORAGE = {
        "android.permission.READ_EXTERNAL_STORAGE",
        "android.permission.WRITE_EXTERNAL_STORAGE" };

public static void verifyStoragePermissions(Activity activity) {

    try {
        //检测是否有写的权限
        int permission = ActivityCompat.checkSelfPermission(activity,
                "android.permission.WRITE_EXTERNAL_STORAGE");
        if (permission != PackageManager.PERMISSION_GRANTED) {
            // 没有写的权限，去申请写的权限，会弹出对话框
            ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,REQUEST_EXTERNAL_STORAGE);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

五、写入数据 

File f2=new File(ext,"test.data");
try {
    verifyStoragePermissions(MainActivity.this);
    OutputStream out=new FileOutputStream(f2);
    out.write("test".getBytes());
    out.close();

} catch (Exception e) {
    e.printStackTrace();
}

 六、读取数据

try {
    InputStream in=new FileInputStream(f2);
    byte[] buffer=new byte[64];
    int len=in.read(buffer);
    String str=new String(buffer,0,len);
} catch (Exception e) {
    e.printStackTr
    ace();
}