How Can I Solve This Error When I Wont Get Backup Of Database ... Help Me

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C#

SQL-Server-2008R2

Cannot open backup device 'I:\aaa.bak'. Operating system error 32(The process cannot access the file because it is being used by another process.).

here my code

private void button_backup_Click(object sender, EventArgs e)
{
//con is the connection string
con.Open();
string str = "USE TestDB;";
string str1="BACKUP DATABASE TestDB TO DISK = 'E:\backupfile.Bak' WITH FORMAT,MEDIANAME = 'Z_SQLServerBackups',NAME = 'Full Backup of Testdb';";
SqlCommand cmd1 = new SqlCommand(str, con);
SqlCommand cmd2 = new SqlCommand(str1, con);
cmd1.ExecuteNonQuery();
cmd2.ExecuteNonQuery();
MessageBox.Show("success");
con.Close();
}