Cannot overwrite file open with FileShare.ReadWrite

Create a simple test app:
private void Form1_Load(object sender, EventArgs e)
{
FileStream fs = new FileStream(@"\sd card\test.txt",
FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite);
}

Try to copy another test.txt file over \sd card\test.txt and get error
message (file being used by another process).

Any ideas?
Thanks