Staging / Empty space omission


This is going to be a re-post from Spiceworks community:
I have about 5 slices. Total combined, non-compressed size is roughly 750GB. One slice is storage, and has roughly 3tb, with ~640GB data.

I've ran into a bit of a tough pickle there. Our backup server for now is just an old server with 2tb drive. Our new box is bare metal hyperv with 4tb space. However all of our VM's together add up to only about ~700GB. I do daily data-only backup and that rounds out to be about 650gb.. And I want to do a complete VMs backup once a week.

Testing out I've been setting destination on the script to be our storage vm. works like a charm, of course it doubles the size.. anyway..
So I set it to backup to an empty 2tb drive on target server, but it runs out of space before it can complete the backup.

Is there anyway in the command line to force it to omit the empty space?
I guess I could make a sacrificial VM with sole purpose of serving as staging space with enough room for the .netzip to do its thing before moving it to target?

Any thought on this?