Is the VM named "openmediavault"?
It's possible that, if there was a snapshot, that snapshot is still in the process of committing once deleted.
Is there some sort of backup software that takes a snapshot while backing up the VMDK?
If the VM RAM is set at 5GB and Thick provisioned disk is configured for 800GB, and no snapshots are in place, it is very unlikely that there would be more than 805.5GB of disk used (the vm log files and vmx vswp which covers the RAM allocated as overhead to run the machine).