Questions about Converting MBR to UEFI on a HDD That The PC Boots from

I have a problem that is quite unique, since I was unaware at the time that Windows doesn't recognize HDD's greater than 2TB unless using UEFI, it was first a problem installing windows as the PC had 2 hard drives. One was the 4TB HDD. The other, a 256GB M.2 NVMe SSD hard drive.

I ended up installing Windows on the SSD hard drive, but for some reason the computer boots from the 4TB HDD.

I now know that I need to convert the MBR to UEFI in order to get access to the extra 2TB of the hard drive, however, since space was limited on the SSD, I changed the location of "My Documents" (including My Pictures, My Videos etc.) to be on the 2TB partition of the 4TB HDD.

Now my first dilemma is, can I simply copy the files on the 2TB HDD to another hard drive in order to back up everything, or do I need to create an image of the partition before I remove all partitions and then go ahead and change the HDD from MBR to UEFI.

My second dilemma is, will the HDD boot (after I change the BIOS settings from Legacy to UEFI) from the HDD after I've deleted all partitions and changed the HDD from MBR format to UEFI? Or, as I suspect, are there some files in the first partition of the HDD which the computer uses to boot from?

Does any of this make sense, or have I made some mistakes in terminology or my understanding of how things work?


  • +1 vote

    There's a hidden boot partition. If you delete all partitions on the 4TB, it won't boot anymore.

    Why don't you stick to your plan to use 256GB as your OS drive and the 4TB for your data? In BIOS, you need to change the boot to UEFI and boot option priorities though. Booting from a SSD is soooooo much faster.


    Yes that's what I wanted to do - but Windows was giving me error messages, I think now because i was booting into legacy mode instead of UEFI mode.

    It was a while ago now, so I can't remember the specifics, but I had trouble installing Windows in the first place, which is why I have the setup I currently do. I wanted to avoid starting over from scratch, but perhaps that would be the best way.

    • Can I clone that hidden partition? Or is it impossible to convert a bootable MBR HDD to UEFI?
    • +3 votes

      NVME SSD won't work with legacy boot, needs to be UEFI.

      If you clone all partitions on a drive to another with similar or bigger size, yes, you can clone the hidden partition.

      Start from scratch with the SSD, you will not regret it. Switching to SSD is one the best upgrade one can do.


        I honestly think that would be the best thing to do - but I simply hate reinstalling Windows. In the past with XP I had a disc image of a fresh install with all the programs I wanted on it. That was the life.

  • +1 vote

    There are some files that the computer boots from. I am having trouble finding a suitable guide, but the missing files are the Windows Boot Manager and you change the files with bcdedit


      Thanks for that. I read one guide about using the windows installation USB, then entering the console to fix boot problems, however because I'm unfamiliar with several steps of this process, IE, windows on SSD drive, user files for windows on a different drive - and they wont fit now back onto the SSD, it seems like my best option would be to back everything important up, then start from scratch, boot using UEFI then scrub the entire 4tb HDD and all its partitions that windows made, even though I installed windows on the SSD. If you think that's confusing to read, imagine me trying to untangle it all when I'm no expert.

      • +2 votes

        Yeh, that might be the easiest way. As an idea, you could unplug the 4TB drive, then use the Windows installation USB to "fix" the boot problem so that the computer boots with just the SSD. Then plug in the 4TB.


          That's not a bad thought, I might give that a try, thanks. Problem still lies with the My Documents files, I believe some of those may have configuration files for various programs etc. But it can't hurt to try. Cheers.

          • +1 vote

            @ozchappy: I think you'd have to shrink the existing partition on the SSD so there is space for it to generate a new boot partition and stuff, then yeah repair/install over the top with it in UEFI mode would be my guess to fix it. Though honestly I haven't had to do anything dodgy with Windows like this since 7 or maybe even XP.

            Regardless I'd be backing up everything before you go too deep into it.


              @smashman42: Cheers, a solid suggestion, and of course, backing everything up first goes without saying.

  • +2 votes

    You don't need to repartition or reinstall for the simple case of switching to GPT/UEFI. The MBR2GPT tool will convert a running Win10 installation over to GPT with UEFI boot. Then you just need to flip your motherboard over to UEFI boot and you should be good to go.

    It's an official MS tool and has worked well for me when I needed it for similar reasons.

    That said, ideally you install Windows proper on the SSD. That's somewhat more involved. You could either do a full reinstall or convert the HDD to GPT first (with the tool above), shrink the partition, and then clone the entire now-GPT drive.


      Can I run that tool from the windows install/repair USB console?

      Or are you saying I don't even need to do that? Just open cmd prompt and run the MBR2GPT tool while logged in, that doesn't sound feasible, so I'm guessing it's not that easy.

      • +1 vote

        You can do either.

        MBR2GPT.EXE is located in the Windows\System32 directory on a computer running Windows 10 version 1703 (also known as the Creator's Update) or later. The tool is available in both the full OS environment and Windows PE.

        Windows PE is the install/repair environment.


          Thanks, I'll back everything up and give it a go when I get some spare time.

          If all else fails, simply starting from scratch is probably the best idea anyway, but I won't lose anything by trying to fix my own stuff up first.

          Appreciate the help.