In your controller use:
var path = HttpContext.Server.MapPath("~/Data/data.html");
This allows you to test the controller with Moq like so:
var queryString = new NameValueCollection();
var mockRequest = new Mock<HttpRequestBase>();
mockRequest.Setup(r => r.QueryString).Returns(queryString);
var mockHttpContext = new Mock<HttpContextBase>();
mockHttpContext.Setup(c => c.Request).Returns(mockRequest.Object);
var server = new Mock<HttpServerUtilityBase>();
server.Setup(m => m.MapPath("~/Data/data.html")).Returns("path/to/test/data");
mockHttpContext.Setup(m => m.Server).Returns(server.Object);
var mockControllerContext = new Mock<ControllerContext>();
mockControllerContext.Setup(c => c.HttpContext).Returns(mockHttpContext.Object);
var controller = new MyTestController();
controller.ControllerContext = mockControllerContext.Object;