function DriveExists(DriveByte: Byte): Boolean;
begin
  Result := GetLogicalDrives and (1 shl DriveByte) <> 0;
end;

function DriveType(DriveByte: Byte): String;
begin
  case GetDriveType(PChar(Chr(DriveByte + Ord('A')) + ':\')) of
    DRIVE_UNKNOWN: Result := 'unbekannt';
    DRIVE_NO_ROOT_DIR: Result := 'Laufwerk existiert nicht';
    DRIVE_REMOVABLE: Result := 'Wechselmedium';
    DRIVE_FIXED: Result := 'Festplatte';
    DRIVE_REMOTE: Result := 'Netzwerk';
    DRIVE_CDROM: Result := 'CD-ROM/DVD';
    DRIVE_RAMDISK: Result := 'RAM Disk';
  else
    Result := 'anderer Laufwerkstyp';
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  I: Integer;
begin
  for I := 0 to 25 do
    if DriveExists(I) then
      ListBox1.Items.Add(Chr(I + Ord('A')) + ':\ (' + DriveType(I) + ')');
end;

procedure LoadLogicalDrives(Strings: TStrings);
var
  S: DWORD;
  AllStrings, CurrentString: PChar;
begin
  S := 255;
  GetMem(AllStrings, Succ(S));
  GetLogicalDriveStrings(S, AllStrings);
  try
    if AllStrings <> nil then
    begin
      CurrentString := AllStrings;
      while True do
      begin
        Strings.Add(StrPas(CurrentString));
        Inc(CurrentString, Succ(StrLen(CurrentString)));
        if CurrentString[0] = #0 then
          Break;
      end;
    end;
  finally
    FreeMem(AllStrings);
  end;
end;