Quickie: Disabling the Volume Shadow Copy (VSS) service will cause Mbr2Gpt /convert /allowFullOs to fail.

Disabling the Volume Shadow Copy (VSS) service will cause Mbr2Gpt /convert /allowFullOs to fail. 

In \windows\setupact.log you'll see "Cannot find room for the EFI system partition", but a bit above that, you'll see ...

BCD: BcdGetElementDataWithFlags: Failed to open key. Object: {partition-guid} Type: 14000008 Status: c00000034 

Changing the Volume Shadow Copy (VSS) service to manual start, the default, fixes it.