public static void Main()
{
    string[] paths = new[] { "/a/b", "/a/bb", "/a/bbb", "/b/a", "/b/aa", "/b/aaa", "/c/d", "/d/e" };
    string root = "/";

    Console.WriteLine(string.Join(", ", paths.Select(s => GetSubdirectory(root, s)).Where(s => s != null).Distinct()));
}

static string GetSubdirectory(string root, string path)
{
    string subDirectory = null;
    int index = path.IndexOf(root);

    Console.WriteLine(index);
    if (root != path && index == 0)
    {
        subDirectory = path.Substring(root.Length, path.Length - root.Length).Trim('/').Split('/')[0];
    }

    return subDirectory;
}

var startingPath = @"c:";

var directoryInfo = new DirectoryInfo(startingPath);

var result = directoryInfo.GetDirectories().Select(x => x.FullName).ToArray();

void Main()
{
  string [] paths = {  @"/a/b", @"/a/bb", @"/a/bbb", @"/b/a", @"/b/aa", @"/b/aaa", @"/c/d", @"/d/e" };

  var result = paths.Select(x => x.Split('/')[1]).Distinct();

  result.Dump();
}

var result = paths.Select(x =>x.Split(new string [] {"/"},
                                        StringSplitOptions.RemoveEmptyEntries)[0])
                  .Distinct();

var rootList = new List <string>();

foreach (var fullPath in myPaths)
{
    rootList.Add(Path.GetPathRoot(fullPath))
}

return rootList.Distinct();

myPaths.Select(x => Path.GetPathRoot(x)).Distinct();

myPaths.Select(x => Directory.GetDirectoryRoot(x)).Distinct();

string dir = @"C:Level1Level2;

string root = Path.GetPathRoot(dir);

string pathWithoutRoot = dir.Substring(root.Length);

string firstDir = pathWithoutRoot.Split(Path.DirectorySeparatorChar).First();

string currentDirectory = "/";
var distinctDirectories = paths.Where(p => p.StartsWith(currentDirectory)
                               .Select(p => GetFirstSubDir(p, currentDirectory)).Distinct();


...


string GetFirstSubDir(string path, string currentDirectory)
{
    int index = path.IndexOf('/', currentDirectory.Length);
    if (index >= 0)
        return path.SubString(currentDirectory.Length - 1, index + 1 - currentDirectory.Length);
    return path.SubString(currentDirectory.Length - 1);
}