NIO enigma

I am seeing anomalous behavior on Windows XP Pro in my NIO code. The
code in question is as follows and it works as expected on Linux.

import java.io.ByteArrayInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.channels.Channels;
import java.nio.channels.FileChannel;
import java.nio.channels.ReadableByteChannel;

public class FileAppender {
   public static void main(String[] args) throws IOException {
      FileChannel fs = new FileOutputStream("output",
true).getChannel();
      ByteArrayInputStream bs = new ByteArrayInputStream(new byte[]
{50, 51});
      ReadableByteChannel byteChannel = Channels.newChannel(bs);
      fs.transferFrom(byteChannel, 0, 1024);
      fs.close();
   }
}

As you can see, I am using NIO to append to an existing file. In
Windows, the code always overwrites the first two bytes, whereas in
Linux two bytes are added by each run. Although the code here deals
with a byte array, the same problem happens with any input stream. The
problem is as follows:

On Linux this produces "2323" when run twice. On Windows it produces
"23". Note that I am using Sun's JDK 1.5.0_07 in both cases.

Does anyone have experience with OS specific bugs for NIO? Is there a
known issue around this behavior? I noticed an open bug with Sun on NIO
(http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4950302) where
random write produces correct results on Windows but not on Linux.

Thanks for your help!