Installing USB boot64.efi usb image on linux

[ips]

Dear all

I am running Ubuntu 16.04 64 bit natively on Dell XPS13 ultrabook 930 with i7 7y75, 16gb ram, 512.
I installed the virtualbox from software center and proceeded to install a 64 bit windows image (option of 64 bit chosen), allocating 8gb ram and 64gb hd with dynamic memory management.

I mounted the usbc drive fat32 which contains the windows recovery media created from the original windows home installation on the dell xps13. the windows installation was deleted when linux was installed.

I get the following error message when I try to load an efi file.

Could not get the storage format of the medium '/media/external/efi/boot/bootx64.efi' (VERR_NOT_SUPPORTED).


Result Code:
VBOX_E_IPRT_ERROR (0x80BB0005)
Component:
MediumWrap
Interface:
IMedium {4afe423b-43e0-e9d0-82e8-ceb307940dda}
Callee:
IVirtualBox {0169423f-46b4-cde9-91af-1e9d5b6cd945}
Callee RC:
VBOX_E_OBJECT_NOT_FOUND (0x80BB0001)


I am newbie so not sure how to proceed next. I have gone through some of the general suggestion. I.e. I added a usb controller to the storage and then tried to add first the plain boot.efi image from the recovery usb stick and also the boot64.efi image and get the same error outlined above.

I have uploaded the log file.

Many thanks for all your help.