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I have a PC with Windows 10. Had to replace the motherboard and CPU. Now the PC won't boot from C drive. Error message:

"An operating system wasn't found. Try disconnecting any drives that don't contain an operating system".

The motherboard recognizes the C drive since it shows in BIOS setup as expected.

Downloaded and plugged in Windows 10 Installation Media into USB. Tried to "repair Windows". Result: "Could not repair Windows".

Tried to reinstall Windows from Windows 10 Installation Media without loosing data. Got this error:

"The upgrade option isn't available if you start your computer using Windows installation media.

If a copy of Windows is already installed on this computer and you want to upgrade, remove the installation media and restart your computer. After Windows has started normally, insert the installation media and run Windows Setup."

The problem is that Windows won't "start normally" or at all.

Contacted Microsoft and was promptly advised to "reinstall everything".

In short: I have a PC with a perfectly "healthy" C drive that won't boot. The obvious solution would be to reinstall Windows from scratch. This however will take me days because I have a lot of software development tools installed with plug-ins, special settings, etc. Is there a better way?

  • Hi there! Just wondering what you replaced your failed ASUS Z97-A motherboard with? It sounds like the new board is a different model to the one it replaced. – wrecclesham 2 days ago
  • @wrecclesham The new setup is AMD-based. At first I wanted to get the same mobo as the old one, but only found used ones for more money than I paid 5 years ago (1150 must be out of style these days :). I did get a small board with 1150 socket and tested the CPU to make sure it works. BTW, tried to boot from the same C drive with the same negative result. So just changing the mobo and keeping the CPU won't solve the problem. – user1566515 2 days ago
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    So all I need is a time machine :) Or maybe buy a used Z97 board just to do the migration. However, I won't be surprised if that board is not identical to the one I have in Microsoft's point o view. – user1566515 2 days ago
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    You'd be surprised! If you bought this refurbished ASUS Z97-A, then it would allow you to both use your old CPU and boot into your previous Windows installation as if nothing happened: ebay.com/p/Asus-Z97-A-LGA-1150-Intel-Motherboard/216050153. I once bricked a friend's laptop years ago during a BIOS update that went sideways. Had an exact replacement shipped in from India! Installed it and gave it back to him. He still has no idea and Windows never needed to be reinstalled because the hardware matched! 😂 Dell do this under warranty for my office, no Windows reinstalls needed. – wrecclesham 2 days ago
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    @JW0914 yeah the first thing would be to quickly rule out incorrect BIOS options on the new board, just to see how far he can go with it. But boot order sounds too obvious to have missed and if there's only one drive, the order won't matter. Plus, he's already said he tried legacy mode and both alternatives. Try taking a hard drive out of a computer and putting it inside a completely different machine. How much hope would you have that it would boot normally, even with the right BIOS options? Not much! – wrecclesham 2 days ago
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There are a lot of driver changes and without proper cleanup of old drivers and installation of new drivers, changing the motherboard without reinstalling windows is messy.

However, your question specifically asks about booting the system.

A likely cause of this failure is that you were booting your system in Legacy/MBR mode or UEFI mode prior to the change, and the new motherboard is using the alternate mode.

Go in to your BIOS/UEFI settings. Chances are the motherboard is, by default, trying to boot in UEFI mode. Look for an option to enable Legacy/MBR mode. These options will usually be found in the boot settings menu of your firmware settings. If it is booting in Legacy mode already, then change it to UEFI.

  • 1) Yes, I understand that driver changes are necessary, but I thought they could be done without wiping out all existing data on the drive. 2) Found "Storage Boot Option Control" in BIOS. It has three options: /Disabled/UEFI Only/Legacy Only/. Tried them all. Didn't help, although this was a great idea to try. 3) Yes, the drive is MBR. I understand that converting to GPT will erase all data. Perhaps it's possible to back up and then restore once the disk is GPT? I'm not sure a simple copy/paste will work though. – user1566515 2 days ago
  • @user1566515 if the drive is MBR then you’ll need legacy mode boot. Does the behavior change at all under that mode? In that mode can you try startup repair and report if anything changes? You shouldn’t be getting a missing operating system error in the right boot mode. Instead you might deal with a driver issue where the OS just keeps rebooting / blue screening. Do you have any other drives that are, or were, hooked up? – Appleoddity 2 days ago
  • I don't have any other drives connected. Only the USB stick with "Windows Installation Media" and the C drive. Your suggestion makes perfect sense to me, so I just verified again (in case I missed something earlier). Yes, the behavior doesn't change in Legacy mode. "Repair" doesn't work. When I choose "Upgrade", it shows the error included in the original question. If I choose "Custom: Install Windows Only (advanced)", it complains that the disk is MBR. – user1566515 2 days ago
  • @user1566515 bcdedit would be the tool to use if the partition changed letter for an unknown reason. Not to forget that you might have changed the harddrive controller too, so you might face a BSOD after. – yagmoth555 2 days ago
  • @user1566515 it complains that the disk is MBR just means you're booting the installer (USB) in UEFI mode, that's the problem. Again, if you want to recover the old installation you must 1. Enable Legacy/CSM and 2. boot the installer in the same mode. Choosing "Legacy only" should be enough. – GabrielaGarcia 2 days ago
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The OP's previous motherboard, an ASUS Z-97-A, recently failed and has since been replaced with a completely different model.

This would have been fine had performing a clean install of Windows and all applications been a viable option; however, it has since transpired that reinstalling and reconfiguring the OP's development applications will be challenging and he wishes use different hardware without going the clean-install route.

Replacing the previous board with an absolutely identical model would have avoided this new problem, which is related to HAL (hardware abstraction layer) changes introduced by replacing the old motherboard, leaving the existing Windows installation expecting the previous model.

enter image description here

It is difficult to restore to dissimilar hardware. The problem is that the hardware-dependent Microsoft Hardware Abstraction Layer drivers are embedded throughout the previous Windows system's installation.

There are three main options:

  1. Use the new motherboard and simply reinstall Windows to solve the HAL mismatch issues (undesirable in this case as the applications are complex to configure.) This option will, however, guarantee a perfectly stable system with the new motherboard.

  2. Match the failed motherboard with an exact replacement, allowing you to continue to use the existing Windows installation. Windows won't even notice anything changed and will be perfectly stable. (Difficult as this motherboard is discontinued and a replacement would take time to arrive.)

    • Motherboards and other hardware components can be replaced by running Sysprep on the old system before it fails, in order to make a hardware-agnostic image that can be migrated to different hardware. Unfortunately, the old motherboard has already failed and no such image exists.

    • An alternative to Sysprep would be using a 3rd-party tool, such as EaseUS Todo Backup, to "restore a system to dissimilar hardware." But even specialized 3rd-party tools require an emergency backup to be created prior to the previous hardware failing.

If I was in this situation, I would consider returning the different model board (if possible), and buy an identical one, even if that meant used or open box. The main problem with that is you would have to wait weeks for it to arrive. On one hand, this could save you having to reinstall everything from scratch but waiting that long for a part to arrive might not be an option if this needs to be functional as soon as possible.

It depends if you would rather spend days reconfiguring your applications after a clean install and having the system up and running much faster, or waiting a couple of weeks for an identical board to arrive and having zero configuration of applications.

Most people would probably go for the reinstall because most people's software configurations aren't quite as complex as this. It all comes down to exactly how difficult configuring these applications will be and how much of a delay you are able to tolerate before the workstation is fully operational.

If you are able to get the old system to boot using the new motherboard, that would be excellent, especially in the short term, but it would be a kludge; I wouldn't entirely trust a critical workstation with a different motherboard model if it wasn't either Sysprepped first or received a clean install afterwards.

Trying to patch up an unstable computer with HAL issues is much more frustrating than slowly and methodically setting up a new installation from scratch, at least in my experience!

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    Aside from some manual driver updates in safe mode, I have actually never had issues with hardware swaps, especially with the HAL setting (you'd really have to change the hardware to an entirely different architecture). Besides, you can force a HAL change, even offline by offline registry editing. At the very least I'd say get it bootable, then do an image backup and restore to dissimilar hardware as you suggested. – Jarrod Christman 2 days ago
  • Another idea, he could also plug the drive into another computer, image the drive, and then do a restore...etc. – Jarrod Christman 2 days ago
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    For the past 15 years I've never had a problem with swapping motherboards. I've gone from XP -> Vista -> 7 -> 10 without once wiping the OS either. In the days prior to XP I would have agreed with you but swapping out hardware (even the motherboard) is pretty safe now. If OP's problem does end up being the hardware swap I'd say he's on the rare end, rather than the usual. – MiniRagnarok 2 days ago
  • @JarrodChristman "he could plug the drive into another computer." Unless the new computer was very similar to the old one, it could have the same problems as this motherboard is having now. I don't know how stable it would be if you Sysprepped using the wrong hardware for the initial image. You can ask Microsoft but I'm sure they will say that wouldn't guarantee success because it's not a supported method. The right way to do this is pretty well documented. – wrecclesham yesterday
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    I don’t agree with either commenter about not having issues with the OS after a motherboard change. Sometimes you might be able to get away with this if the hardware is similar enough. Now, I feel pretty confident Windows 10 handles this scenario much better. But, Windows XP would almost never handle this situation. There have been changes to the HAL architecture over the years. It has gotten better but it has been a very serious issue, causing trouble in almost all cases to being less of a problem in Windows 10. – Appleoddity yesterday
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The boot mode (Legacy / UEFI) is not the only BIOS setting that can prevent Windows from starting.

I recently had an issue with a PC that wouldn't boot Windows anymore. The BIOS battery died, and it wouldn't keep the settings anymore. Windows Boot Manager would start, but Windows would crash and start the boot repair, which would fail constantly.

I exchanged the BIOS battery, and had a look at the SATA settings in the BIOS. By default it was switched to Legacy (IDE) mode. Changed the setting back to the newer AHCI mode, and Windows booted again without a glitch.

You should check that on your computer, depending on what it was before, then it might be either Legacy or AHCI.

Depending on your PC and BIOS model, this entry can usually be found under Advanced Settings or Integrated Peripherals, and then SATA settings / Disk controller settings / IDE settings.

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I suggest a reinstall because you changed the motherboard. A lot of driver change would happen, even if it would had boot a clean install would remove anything there from the old mainboard.

Next time if you want to prevent a full reinstall you would had to run a sysprep inside Windows before installing the new mainboard. So its a last command you issue and you close the OS.

sysprep remove know device and when the PC restart it start the redetection like when it install.

  • A driver issue would not result with the BIOS/UEFI Firmware not handing off to the Windows bootloader. Driver issues after booting would be Chipset and IMEI related, of which cannot be obtained through Windows Update since they must be installed prior to any software or Windows Updates. Running SysPrep would not be recommended in this situation for a myriad of reasons (less invasive solutions exist, data loss, potential software license loss/inconvenience if locked to specific OS IDs, etc.) – JW0914 2 days ago
  • @JW0914 Like I stated to the OP; "@user1566515 bcdedit would be the tool to use if the partition changed letter for an unknown reason. Not to forget that you might have changed the harddrive controller too, so you might face a BSOD after. " Anyhow I still think that a re-install would be faster and more conveniant in the end, as it would be up and running in less than 30m, except if on ebay he get the same motherboard model – yagmoth555 2 days ago
  • Why would bcdedit or bootrec be utilized when the HDD boot order would simply need to be modified in the BIOS/UEFI firmware? The only reason a clean install would need to be performed is if experiencing issues due to a Chipset and/or IMEI driver mismatch, since CPU related drivers must be installed prior to any other software or Windows Updates and are not available through Windows Update. – JW0914 2 days ago
  • @JW0914 bcdedit is not there to modify the boot order, it's used when the drive letter does not match, thus to use to fix when the bootloader doesnt find the boot partition (like the OP state, OS not found) it got nothing to do with the boot order. – yagmoth555 2 days ago
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    This is a common issue and there are zero reasons to do anything software-wise, as the issue is not software related. The issue arises either from the boot order of the HDDs (due to not plugging the HDD with the C: partition into SATA0, so either the connection needs to be corrected on the MB or boot order changed in the BIOS/UEFI Firmware) or if UEFI Firmware, Windows was originally installed in EFI mode with the MB set to Legacy/MBR mode or vice versa. Current suggestions are an over-complication of the issue, as there are less-invasive and more efficient solutions. – JW0914 2 days ago

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