Shrinking a Virtual Disk on Storage Spaces

Shrinking a Virtual Disk on Storage Spaces

I have a Windows 2012 Server with some disks, which i used to create a
couple of storage spaces. I originally allocated 350Gb to a volume, which
is now more than i need. I can shrink the Volume that Windows sees, which
managed to bring down to about 200Gb, but that 150Gb is listed as
"Unallocated" but i cant figure out how to reduce the size of the virtual
disk... What am I missing? The only option i can think is to copy the
contents of the volume to somewhere else, delete the volume and re-create
it smaller... but that's a bit of a pain...
i have tried the PowerShell command as suggested here, but i get the same
error (Not supported!). It seems weird that i can make it larger, but not
smaller... the space in the VM is not being currently "used"...
Finally, its a Fixed size Virtual Disk... if that makes any difference?
Thanks.