[megaloman]

Код:

@Echo Off
cls
	For /F "usebackq delims=" %%a In (`"WMic LogicalDisk Where "Caption^='C:'" Get FreeSpace /Value | FindStr.exe /C:"^=""`) Do Set "%%a"
	Set "FSize1=%FreeSpace%"

	......
	......

	For /F "usebackq delims=" %%a In (`"WMic LogicalDisk Where "Caption^='C:'" Get FreeSpace /Value | FindStr.exe /C:"^=""`) Do Set "%%a"
	Set "FSize2=%FreeSpace%"

	Set /A Delta=%FSize2:~0,-6%-%FSize1:~0,-6%
	Echo %FSize2%
	Echo %FSize1%
	Echo %Delta%
pause
Exit /B 0

Результат не в мегабайтах, а в миллионах байт.