I was doing the instructions at https://apple.stackexchange.com/a/239203/7125
When after doing "bcdboot e:\windows /v /s e:" I get this error:
BFSVC Error: SetNamedSecurityInfo failed! Error code = 0x5
BFSVC Error: No process found using \\?\GLOBALROOT\Device\HarddiskVolume5\EFI\Microsoft\Boot file.
BFSVC Error: BfspSetSecurityDescriptor(\\?\GLOBALROOT\Device\HarddiskVolume5\EFI\Microsoft\Boot) failed! Last Error = 0x5
There's some other text around the error, but this seems to be the important part.
The external HDD is in a SATA USB 3 enclosure, if that matters. I'm executing bcdboot from a functioning Win 10 installation in a VM, with the external HDD connected as E: