type
  TDriveType = (
    dtUnknown,
    dtNoRootDir,
    dtRemovable,
    dtFixed,
    dtRemote,
    dtCDROM,
    dtRAMDisk
  );
  TDriveTypes = set of TDriveType;

procedure GetDriveList(Drives: TStrings; Filter: TDriveTypes = []);
var
  P: PChar;
  Buffer: string;
  BufLen: Integer;
begin
  BufLen := GetLogicalDriveStrings(0, nil);
  if BufLen > 0 then
  begin
    SetLength(Buffer, BufLen);
    if GetLogicalDriveStrings(Length(Buffer), PChar(Buffer)) = 0 then
      RaiseLastOSError;

    P := PChar(Buffer);
    while StrLen(P) > 0 do
    begin
      if (Filter = []) or (TDriveType(GetDriveType(P)) in Filter) then
        Drives.Add(P);
      P := StrEnd(P) + 1;
    end;
  end
  else
    RaiseLastOSError;
end;

// possible usage to list all drive types
GetDriveList(ComboBox.Items);
// possible usage to list fixed drive types
GetDriveList(ComboBox.Items, [dtFixed]);
// possible usage to list fixed and removable drive types
GetDriveList(ComboBox.Items, [dtFixed, dtRemovable]);