The following example demonstrates getting a list of the available logical drives available on the system.

procedure TForm .Button Click(Sender: TObject);
var
  ld : DWORD;
  i : integer;
begin
  ld := GetLogicalDrives;
  for i := 0 to 25 do begin
    if (ld and (  shl i)) <> 0 then
      Memo .Lines.Add(Char(Ord('A') + i) + ':\');
  end;
end;