// Global Varible
var
 GByteAmount: int64;
 GSourceSize: int64;
 GStartTime: Cardinal;
//...........
function TFrmProgressBarMoveFile.CheckFreeSpace(RequestSpace: int64; Drive: String): Boolean;
var
 FreeByte, DriveTotalSpace, DriveFreeSpace, DFreeSpace: Int64;
 CountReadSecond: Int64;
 LastBuffer: Boolean;
 TimeStep  : Cardinal;
begin
 Result     := false;
 LastBuffer := false;
 if Not GetDiskFreeSpaceEx(PChar(Drive), FreeByte, DriveFreeSpace, @DriveFreeSpace) then Exit;
 DFreeSpace := DriveFreeSpace;

 // check the last buffer
 if (GByteAmount <> 0) and ((MOVE_BUFFER_SIZE * DEFAUT_BUFFER) >= (GSourceSize - GByteAmount)) then LastBuffer := true;

 TimeStep := ((GetTickCount - GStartTime) div 1000);

 if LastBuffer or ((GByteAmount > 0) and (TimeStep > 0)) then
 begin
   if DFreeSpace > RequestSpace then Result := true;
 end
   else
 begin
   CountReadSecond := Trunc(GByteAmount / TimeStep);
   if (RequestSpace + CountReadSecond) < FreeSpace then Result := true
 end;

end;

if LastBuffer or ((GByteAmount > 0) and (TimeStep > 0)) then