Here's my disk layout, I will be adding an EFI System Partition to my Disk 0. You will have to adjust a few of the commands to match your drive # and drive letter of the Windows you want to boot.
I ran these commands in a Command Prompt (Admin). I believe they will also work in a PowerShell (Admin):
Code:
Microsoft Windows [Version 10.0.10586]
(c) 2015 Microsoft Corporation. All rights reserved.
x:\windows\system32>diskpart
Microsoft DiskPart version 10.0.10586
Copyright (C) 1999-2013 Microsoft Corporation.
On computer: MININT-KDT67KV
DISKPART> list disk
Disk ### Status Size Free Dyn Gpt
-------- ------------- ------- ------- --- ---
Disk 0 Online 447 GB 7168 KB *
Disk 1 Online 931 GB 1024 KB *
Disk 2 No Media 0 B 0 B
DISKPART> select disk 0
Disk 0 is now the selected disk.
DISKPART> list part
Partition ### Type Size Offset
------------- ---------------- ------- -------
Partition 1 Reserved 16 MB 1024 KB
Partition 2 Primary 447 GB 23 MB
DISKPART> select part 2
Partition 2 is now the selected partition.
DISKPART> shrink desired=100
DiskPart successfully shrunk the volume by: 100 MB
DISKPART> create partition EFI
DiskPart succeeded in creating the specified partition.
DISKPART> format fs=fat32 quick
100 percent completed
DiskPart successfully formatted the volume.
DISKPART> assign letter=z
DiskPart successfully assigned the drive letter or mount point.
DISKPART> exit
Leaving DiskPart...
x:\windows\system32>bcdboot C:\Windows /s z: /f UEFI
Boot files successfully created.
x:\windows\system32> exit
Notes about some of the commands:
Select Disk # <- Select the disk # of the disk you want to make bootable.
Select Part # <- Select the partition # of the Windows partition that is occupying all the space on the drive
bcdboot C:\Windows /s z: /f UEFI <- Change the C: to the current drive letter assigned to the Windows that you want to boot.
After the commands are completed, you will probably need to enter you UEFI setup when restart your computer to set it to boot from the new Windows Boot Manager on the drive you want to boot from. After you are boot from the desired drive, if the new EFI System Partition has a drive letter assigned to it, you can (and should) remove that drive letter from it in disk management.