'error: invalid signature' when booting Windows 10

[David Collier]

I am trying to retire an old HD where grub /boot directory happens to
reside, so I am trying to move it to the current Linux root partition which
is on an SSD.

this is the script used to provide a Windows boot option in the Grub menu:

vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
#!/bin/sh
exec tail -n +3 $0
menuentry "Windows 10" {
   insmod fat
   insmod chain
   insmod part_gpt

   set root=(hd1,gpt3)
   chainloader /efi/Microsoft/Boot/bootmgfw.efi
}
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

the problem is that when trying to pick this option from grub menu when
booting from the new location, it reports 'error: invalid signature'. The
same menu worked fine when it was booting from the old disk (I made double
checked that (hd1,gptr) points at the correct partition).

I searched this list and found this thread:
http://lists.gnu.org/archive/html/help-grub/2016-05/msg00020.html but am
not quite sure how to apply this to the Windows case.

TIA!

-dc