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My issue is different from the other people who have had this issue.

I used to dual boot Windows 10 and Debian 9. A Windows update changed the boot loader and I was unable to boot either. A NeoSmart product let me boot into Windows 10 but the Debian 9 partition was lost. I moved Window 10 to a new drive for added space. I'd like to SecureBoot so I can upgrade to Window 11 but I think the NeoSmart loader uses MBR (Master Boot Record). Now I'm seeing the BIOS logo, then a black screen with a "1234F:" prompt that doesn't respond to anything I type on my USB keyboard. Pressing the power button does not turn the computer off, it only reboots back into the prompt.

How can I make my computer bootable again, with a single, SecureBoot to Windows 10?

Unlike everyone else, I do not need to dual boot. I would like to boot with the official Windows bootloader. I have an old laptop that dual boots Widows 10 and Debian 10 with GRUB. I also have a laptop that boots Windows 11 (preinstalled). The non-booting computer has an external, USB, optical drive. I think it has a Gigabyte X570 AORUS Ultra motherboard with an Rizon 9 CPU.

Dave Brunker
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    “How can I make my computer bootable again, with a single, SecureBoot to Windows 10?” - Confirm you are actually booting in Legacy mode, then use, MBR2GPT a tool built-into Windows 10+; Without knowing if your actual booting while in Legacy Mode it’s impossible to answer this question. Here is an answer with instructions. It’s probably a duplicate – Ramhound Jan 31 '23 at 02:28
  • Usually the way I check is to run Windows Update and it tells me I meet all the requirements for Windows 11, except booting with SecureBoot.

    It's not a duplicate if I can't even boot into a command prompt.

     – Dave Brunker Jan 31 '23 at 03:06
  • “A NeoSmart product let me boot into Windows 10 but the Debian 9 partition was lost.” - I only suggested my solution because you mentioned you could boot into Windows. How did you go from being able to boot into Windows to being unable to? I presume you didn’t install the Windows install originally? – Ramhound Jan 31 '23 at 03:13
  • I wish I had the answer to that. I used the computer in the morning and it was fine. I turned it on in the afternoon and saw a "1234F:" prompt. It seems like 1234F: is a rare issue but from what I've researched it's related to MBR corruption. – Dave Brunker Jan 31 '23 at 03:46
  • @Ramhound Thank you, for your help. It didn't fix the issue directly but it gave me the clues I needed to get my computer working again. You also helped me figure out how to convert from MBR to Secure Boot. I've been trying to figure out how to do this for months and I couldn't have done it without you. – Dave Brunker Feb 02 '23 at 21:42

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The BIOS in some motherboards have a safety feature were the BIOS settings will reset to defaults if the computer shuts down abnormally. That's so overlockers won't put their computer into a state where it can't be recovered (a.k.a. bricked). If you get a black screen with a "1234F:" prompt and you have more than one storage device in your computer, make sure the boot order hasn't changed in the BIOS. You may have set the BIOS settings for the boot drive to C: but the default for the boot drive could be lower (e.g. D:, E:, F:, etc.) and now the computer is trying to boot from a non-booting storage device.

Dave Brunker
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