SysInternals sdelete: zero wipe free space is called -z instead of -c
Posted by jpluimers on 2016/09/20
In the 2009 past, sdelete used the -c parameter to zero wipe clean a hard drive and -z would clean it with a random pattern.
That has changed. Somewhere along the lines, -c and -z has swapped meaning which I didn’t notice.
This resulted in many of my virtual machines image backups were a lot larger than they needed to be.
The reason is that now:
-cdoes a clean free space with a random DoD conformant pattern (which does not compress well)-zwrites zeros in the free space
Incidently, -c is a lot slower than -z as well.
TL;DR: use this command
sdelete -z C:
Where C: is the drive to zero clean the free space.
–jeroen
The Wiert Corner - irregular stream of stuff 
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