UnauthorizedAccessException: Access to the path 'C:\' is denied.

Cant figure out why i’m getting this exception, my c:\ disk has complete read/write permissions for all users and running UNITY as an administrator does not help so I assume its not a permissions issue.
full error:

UnauthorizedAccessException: Access to the path 'C:' is denied.
System.IO.FileStream…ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, Boolean anonymous, FileOptions options) (at /Applications/buildAgent/work/c514da0c8183631c/mcs/class/corlib/System.IO/FileStream.cs:259)
System.IO.FileStream…ctor (System.String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize)
(wrapper remoting-invoke-with-check) System.IO.FileStream:.ctor (string,System.IO.FileMode,System.IO.FileAccess,System.IO.FileShare,int)
System.IO.File.Create (System.String path, Int32 bufferSize) (at /Applications/buildAgent/work/c514da0c8183631c/mcs/class/corlib/System.IO/File.cs:135)
System.IO.File.Create (System.String path) (at /Applications/buildAgent/work/c514da0c8183631c/mcs/class/corlib/System.IO/File.cs:130)
System.IO.File.WriteAllBytes (System.String path, System.Byte bytes) (at /Applications/buildAgent/work/c514da0c8183631c/mcs/class/corlib/System.IO/File.cs:594)
Program.Start () (at Assets/Scripts/Program.cs:28)

code:

using System;
using System.IO;
using System.Net;
using UnityEngine;

public class Program : MonoBehaviour
{
    public void Start()
    {
        System.Net.ServicePointManager.ServerCertificateValidationCallback += (s, ce, ca, p) => true;


        // Create web client simulating IE6.
        using (WebClient client = new WebClient())
        {
            client.Headers["User-Agent"] =
            "Mozilla/4.0 (Compatible; Windows NT 5.1; MSIE 6.0) " +
            "(compatible; MSIE 6.0; Windows NT 5.1; " +
            ".NET CLR 1.1.4322; .NET CLR 2.0.50727)";

            // Download data.
            byte[] arr = client.DownloadData("https://docs.google.com/file/d/0B_jZsNBM3OCeX21oaEZzUTh0enM/edit?usp=sharing");

            // Write values.
            Debug.Log("--- WebClient result ---");
            Debug.Log(arr.Length);

            File.WriteAllBytes("C:\\", arr);
        }
    }
}

If you use File.WriteAllBytes you have to specify a file path. You are trying to write bytes directly to a “directory” which is not possible. This problem will be fixed if you specify a valid file path.