[c#] Server.MapPath - Physical path given, virtual path expected

I'm using this line of code:

var files = Directory.GetFiles(Server.MapPath("E:\\ftproot\\sales"));

to locate files in a folder however I get the error message saying that

"Physical Path given but virtual path expected".

Am new enough to using System.IO in C# so I was wondering if it's possible to enter a physical path to do this?

var files = Directory.GetFiles(@"E:\ftproot\sales");