Hi
 
No, he's talking filling with zeros unused blocks. That's a prerequisite
to optimize for a sparse image _on the other side_ and/or compression
during the transfer.

I know of an ext2/3/4 utility that does precisely that. I have no idea
if such an utility exists for NTFS. 

 
It exists for NTFS, have a look at "sdelete.exe" with -c option.

Momo.