Comment 47 for bug 1396379

The same issue.

Ubuntu broke my Windows EFI system partition, installing Ubuntu EFI on it, despìte I have told Ubuntu to use the external drive.

*To fix the Windows EFI system partition:*

* Boot on Windows CMD using a Recovery Partition/USB Drive
** Windows Bootloader Recovery -> System Restore – > Troubleshoot-> Command Prompt
diskpart
list disk
list partition
list volume
sel disk 0 (Select the disk with the Windows EFI system partition)

Assign the drive letter K: to the hidden EFI volume:
select volume 1 (select the volument with the Windows EFI system partition)
assign letter K:
exit

cd /d K:\efi\microsoft\boot\
attrib BCD -s -h -r
ren BCD BCD.bak
bcdboot C:\Windows /l en-us /s k: /f ALL

*Don't use this commands, because this are only for MBR not UEFI mode:*
* bootrec /fixboot
* bootrec /scanos
* bootrec /rebuildbcd
* or even:
* bootrec /FixMbr