Format of extended free space structure:
Offset	Size	Description	)
 00h	WORD	(ret) size of returned structure
 02h	WORD	(call) structure version (0000h)
		(ret) actual structure version (0000h)
 04h	DWORD	number of sectors per cluster (with adjustment for compression)
 08h	DWORD	number of bytes per sector
 0Ch	DWORD	number of available clusters
 10h	DWORD	total number of clusters on the drive
 14h	DWORD	number of physical sectors available on the drive, without
		  adjustment for compression
 18h	DWORD	total number of physical sectors on the drive, without
		  adjustment for compression
 1Ch	DWORD	number of available allocation units, without adjustment
		  for compression
 20h	DWORD	total allocation units, without adjustment for compression
 24h  8 BYTEs	reserved