It was failing to create files below the machines\default\hyperv folder, rather it was creating folders named 'hypervSnapshots', 'hypervVirtual Hard Disks', and 'hypervVirtual Machines'. This was causing the files to not be removed when destroying the vm, and an error when subsequently bringing it up again.
Implemented the differencing disk for vmcx.
This means the disk is now copied by Hyper-V (Powershell) instead of Ruby for new machines.
This does mean EFI Firmware now does work for machines since it is quite a feep copy. Compare-VM will report incompatibilities should they be found.