Display Directory (Folder) structure for folder outside the project in ASP.Net TreeView control

Here I have created sample that will help you out.

HTML

<div>
    <h3>
        Vehicle Details</h3>
    <hr />
    <asp:TreeView ID="TreeView1" runat="server" ImageSet="XPFileExplorer" NodeIndent="15">
        <HoverNodeStyle Font-Underline="True" ForeColor="#6666AA" />
        <NodeStyle Font-Names="Tahoma" Font-Size="8pt" ForeColor="Black" HorizontalPadding="2px"
            NodeSpacing="0px" VerticalPadding="2px"></NodeStyle>
        <ParentNodeStyle Font-Bold="False" />
        <SelectedNodeStyle BackColor="#B5B5B5" Font-Underline="False" HorizontalPadding="0px"
            VerticalPadding="0px" />
    </asp:TreeView>
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DriveInfo[] drives = DriveInfo.GetDrives();
        DirectoryInfo[] dirInfo = null;
        DirectoryInfo rootInfo = null;
        foreach (DriveInfo item in drives)
        {
            if (item.IsReady)
            {
                dirInfo = item.RootDirectory.GetDirectories("Vehicles");
                if (dirInfo.Length > 0)
                {
                    rootInfo = dirInfo[0];
                    break;
                }
            }
        }
        this.PopulateTreeView(rootInfo, null);
    }
}

private void PopulateTreeView(DirectoryInfo dirInfo, TreeNode treeNode)
{
    foreach (DirectoryInfo directory in dirInfo.GetDirectories())
    {
        TreeNode directoryNode = new TreeNode
        {
            Text = directory.Name,
            Value = directory.FullName
        };

        if (treeNode == null)
        {
            //If Root Node, add to TreeView.
            TreeView1.Nodes.Add(directoryNode);
        }
        else
        {
            //If Child Node, add to Parent Node.
            treeNode.ChildNodes.Add(directoryNode);
        }

        //Get all files in the Directory.
        foreach (FileInfo file in directory.GetFiles())
        {
            //Add each file as Child Node.
            TreeNode fileNode = new TreeNode
            {
                Text = file.Name,
                Value = file.FullName,
                Target = "_blank",
                NavigateUrl = (new Uri(Server.MapPath("~/"))).MakeRelativeUri(new Uri(file.FullName)).ToString()
            };
            directoryNode.ChildNodes.Add(fileNode);
        }

        PopulateTreeView(directory, directoryNode);
    }
}

Screenshot