Unmount USB Drive/Sdcard programatically Android (Have Root Access)

Question

I am trying to unmount the sdcard/usb drive programatically from Android JellyBean device. I have the mountpaths for them. /mnt/udisk and /mnt/extsd respectively. I have SuperUser Access. I have tried running the following code but does not give me the results:

           Process su = null; 
            try { 
                su = Runtime.getRuntime().exec("su");
                String cmd = "umount /mnt/extsd/"+ "\n";
                su.getOutputStream().write(cmd.getBytes());
                String exit = "exit\n";
                su.getOutputStream().write(exit.getBytes());
                su.waitFor();
                Toast.makeText(MyAndroidAppActivity.this, "Completed", Toast.LENGTH_LONG).show();
            } catch (Exception e) {
                e.printStackTrace();
                Toast.makeText(MyAndroidAppActivity.this, e.toString(), Toast.LENGTH_LONG).show();
            } finally { 
                if (su != null) { 
                    su.destroy(); 
                }