how to Dual Boot Windows 10 and Windows Server WITHOUT INSTALLATION

Several internet sites describe how to install Windows Server as a second OS. This is not my case.

On the same PC I have a working drive with Windows 10 Pro and another drive with Windows Server 2012 R2 (+ Visual Studio 2017 + SQL Server 2017 + SharePoint 2016 ...). Both drives with MBR (not GPT). I can boot either one as a single system (drive).

I now want to add Windows Server as a Dual Boot option. This used to work as Dual Boot but was broken while experimenting with a new Ubuntu installation. My previously working Boot Manager/Loaders setup was:

Windows Boot Manager
--------------------
identifier {bootmgr}
device partition=\Device\HarddiskVolume1
description Windows Boot Manager
locale en-US
inherit {globalsettings}
default {current}
resumeobject {4c702cd7-4348-11e9-a123-a126b58ebe81}
displayorder {current} {23a79a8c-7591-11e8-a5b9-10c37b6b0c8f}
toolsdisplayorder {memdiag}
timeout 30
Windows Boot Loader
-------------------
identifier {current}
device partition=C:
path \WINDOWS\system32\winload.exe
description Windows 10
locale en-US
inherit {bootloadersettings}
recoverysequence {4c702cda-4348-11e9-a123-a126b58ebe81}
displaymessageoverride Recovery
recoveryenabled Yes
allowedinmemorysettings 0x15000075
osdevice partition=C:
systemroot \WINDOWS
resumeobject {4c702cd7-4348-11e9-a123-a126b58ebe81}
nx OptIn
bootmenupolicy Standard
hypervisorlaunchtype Auto
Windows Boot Loader
-------------------
identifier {23a79a8c-7591-11e8-a5b9-10c37b6b0c8f}
device partition=S:
path \windows\system32\winload.exe
description Windows Server 2012 R2
locale en-us
inherit {bootloadersettings}
allowedinmemorysettings 0x15000075
osdevice partition=S:
systemroot \windows
resumeobject {23a79a8b-7591-11e8-a5b9-10c37b6b0c8f}
nx OptOut
detecthal Yes

How can I re-establish (or repair) a working Dual Boot environment? ...obviously without installing a fresh Windows Server OS and all the ensuing work on it that followed.

EDIT:In order to recover the Windows Server drive, I issued the following commands yesterday WITH ONLY THE Windows Server DRIVE INSTALLED in the PC:

bootrec /FixMbr
bootrec /FixBoot
bootrec /RebuildBcd

That made Windows Server bootable as a single drive again (and maybe broke the Dual Boot logic from the Windows 10 drive?)

UPDATE:I booted the DVD prepared by Windows 10 with MediaCreationTool1909.exe:

>bootrec /FixMbr
>bootrec /FixBoot
Access is denied.
>bootrec /RebuildBcd
Successfully scanned Windows installations.
Total identified Windows installations: 0
The operation completed successfully.

As before, I have two drives - either of which can be booted directly by changing the Boot Priority order in the BIOS: either Windows 10 or Windows Server 2012 R2 can be booted directly.

The only Dual Boot option that works is the first (default) to Windows 10. Choosing the second option (Windows Server 2012 R2) leads to this screen:

Windows Boot failed

My question is interesting IMHO - anyone can build a new OS on a dedicated drive for test purposes and then decide to "integrate" it into a Dual/Triple/... Boot option. My suspicion is that the three bootrec commands only work on single boot drives. Up to now I have not seen this answered on the internet - all suggest a new install for the second OS to build a Dual Boot option.

SUCCESS & CONCLUSION:see my answer below

2 Answers

If you still have a Windows Server install medium, boot from it, and instead of reinstalling Windows Server, choose to Repair. Go to Advanced options, then Command line, and use the "diskpart" set of commands to list your volumes and partitions, just to make sure they're all accounted for.

If that's the case, exit diskpart (type exit and press Enter), then type:

bootrec /fixboot (Enter)

If no error message, type:

bootrec /fixmbr (Enter)

Then:

bootrect /rebuildbcd (Enter)

And exit the installation/repair setup, then reboot.

FWIW, I too have Windows 10 and Windows Server dual-booting on my PC, and my bootloader looks like this, as displayed in EasyBCD (freeware for personal use):

*There are a total of 3 entries listed in the bootloader.

Default: Windows 10

Timeout: 3 seconds

EasyBCD Boot Device: D:\

Entry #1 Name: Windows 10 BCD ID: {current} Drive: C:\ Bootloader Path: \Windows\system32\winload.exe

Entry #2 Name: Windows Server BCD ID: {ac735421-add0-11e9-9ef7-e94eceb02e15} Drive: F:\ Bootloader Path: \Windows\system32\winload.exe

Entry #3 Name: Safe Mode BCD ID: {ac735424-add0-11e9-9ef7-e94eceb02e15} Drive: F:\ Bootloader Path: \Windows\system32\winload.exe*

As you can see, the main bootloader is set on the D drive, because that's where I installed Windows 10 AFTER installing Windows Server 2019. Windows 10 installed a secondary bootloader during install.

SUCCESS & CONCLUSION:

I finally dared to try EasyBCD 2.4 with the following steps:

BCD Backup/Repair
Backup Settings
Re-create/repair boot files
Perform Action

and Dual Boot works now. But why trust a "black box" (meaning EasyBCD) solution? What did it do?

Answer: it changed...

displaymessageoverride Recovery

in the default (Windows 10) boot loader to:

displaymessageoverride StartupRepair

which suggests that it was fixed by Microsoft "magic". (I admit EasyBCD may have performed other actions too.)

It leaves one question (that I hinted at earlier) unanswered: what if one creates a new single boot OS drive and later on decides to integrate it into a Dual Boot option? I have no idea how to create a Dual/Triple/... Boot from two or more drives (without reinstalling the second OS). I think this is undocumented(?)

I give credit to Moab for the solution, who suggested using EasyBCD.