how to install windows from external bootable disk (not usb flash disk)


NOTE: I believe this only works with 'uefi' bios mode boot.

HOW TO SPLIT DISK IN TWO PARTITION (SHRINK EXTERNAL DISK)
if you have external bootable disk, say 500gb, use disk management to shring disk
(skip this if you have already two partitions on your external disk, 
or if you want to delete files on the disk and use the whole partition to create bootable disk.)
Be careful not to select the wrong disk. Check drive letters in windows explorer (don't mess with your system,
which is usually disk 'c:')

-start/this pc/manage/disk management
-locate the row that says disk 1 (or disk 2, depening on the number of disks in your systme)
-right click on the disk marked with blue part, select shrink disk
-enter amount by which to shrink the disk (enter 10000 which is little less than 10gb, click on shrink)
-right click on the newly created partition (unallocated)
-select new simple volume, set file system to 'fat 32', quick format, next


FORMAT SECOND PARITION OF THE DISK (10 GB IN SIZE)

WARNING: FORMATTING THE PARTITION WILL DELETE ALL FILES FROM IT!
CHECK PARTITION SIZE BEFORE CLICKING ON START format.

if you haven't already, right click on the part of the disk you want to format, 
-select 'format'
-choose 'fat32' as file system
-check 'quick format'
-click on start
-external partition will get a letter, say G:



MOUNT ISO AS DISK IMAGE IN WINDOWS:
Mount The Windows 8 ISO Image Into A Virtual Drive
If you are on windows 8.1 or windows 10:
-right click on windows 10 iso , 'open with', 'windows explorer'
-you will get windows iso mounted as new disk disk drive, seen on the left in windows explorer
-ISO will be assinged a letter, say H:
If you do not have windows 8.1 or windows 10, use 'power iso', 'daemon tools lite', 'virtual cd', or some other virtual cd manager.


MAKING EXTERNAL DISK BOOTABLE:
run cmd as administrator:
start/ type cmd, righ click on 'cmd', select run as administrator, 'yes'

now, remember, on my PC it is:
hdd external partition: 'G:'
ISO: 'H:'

CHECK YOUR SETTINGS. external partition will get a letter, previous to iso image.

still in cmd screen, go to your iso disc, then to its boot directory:
type 'H:', hit enter (without quotes)
type 'cd boot', hit enter (without quotes)
now we need to copy something to external disk (G:): 
MAKE SURE WHICH LETTER IS EXTERNAL DISK!
bootsect.exe /nt60 G:
you should get message  'Successfully updated FAT32 filesystem bootcode.'



COPYING FILES TO EXTERNAL DISK:

Now, copy all files from iso (H:) to external partition (G:)
you can use windows explorer (click on the H: in the left part of windows explorer),

or use command line:
xcopy H:\*.* G: /e /f /h


UNMOUNTING ISO
right click on 'H:' drive in explorer, select 'eject'.

to umount external disk, use icon in the systray, like with any other usb device.


source: http://bootableusb.net/make-bootable-external-hdd-install-windows-78/