'Howto' Recover a bootable device that is MBR when you are now UEFI: MBR>GPT Win 10

My computer has been a little unstable recently (looks like the GPU will need an RMA) and this afternoon I turned on the monitor to a lovely blue screen stating that the BCD entries were corrupt and I should try Win10 installation media. Crikey!

Didn't really expect it as only had one unexpected restart before, but hey ho, off I went in search of a blank USB key. This proved fruitless as although I bought 2 32GB sticks in December, they've both gone walkies. Long story short, I did find another 32GB Stick, and cleared it with my laptop and used the Windows 10 Installation Media creator (I believe I googled ISO tool) to eventually create windows installation media of it.

So did the repair work? Well no. Startup repair failed consistently from the official installation media and I couldn't even re-install windows as you can't upgrade from outside the booted OS you want to upgrade, and I couldn't install alongside my current install on my SSD as apparently I was using EFI boot mode (news to me) and my Win10 SSD install was an MBR disk. *sigh*

Now it appears the reason for this is my recently-updated Ryzen BIOS only supports 2 boot "modes" now: UEFI and UEFI+Legacy. I believe when I originally installed Win10 on this PC I used Legacy and to be honest it may even have been a previous motherboard. It looked like I was not going to be able to set the BIOS now to Legacy to try and boot or install to the MBR device.

Resigned that all easy options were closed to me (and with a 2 day old backup) I went to repartition and reformat a 2TB Sata HDD I had recently made available, and install Win10 on that. This took quite a long time, it was stuck at preparing files at around 80% for ages, something I don't remember from previous windows installs.

Eventually I was back in a bootable, if slow, windows 10 environment and considered my options. Recover from 2 day old backup: valid, but not going away any time soon so on the backburner. Try and recover the bootable bit of the SSD: tricky but the best option I could see.

I thought I would have a go at converting the SSD to GPT and then re-installing the BCD and maybe the bootloader. I used the open source gptgen.exe (v1.1) to convert the drive, and used EasyBCD (which wasn't very happy with either my installations or GPT) to recover the bootloader and BCD info to the SSD. Then I tried to boot from it.

Some progress - not an immediate blue screen but one slightly later on in the boot process which suggested my storage drivers were not ACPI compliant (they are!) and suggested a reinstall. Err - no thanks.

My options here as I saw it were to install windows alongside the windows install on my SSD and then reinstall all my programs, or use the backup. I didn't like either of these, and I had some time to kill and didn't want to rush, so I looked online to see if I had done the boot from GPT thing properly. A lucky link to a forum thread about 3 pages into my google results led me to this fantastic article:

Microsoft TECHNET Converting Windows BIOS installation to UEFI https://social.technet.microsoft.com/wiki/contents/articles/14286.converting-windows-bios-installation-to-uefi.aspx

I am honestly so happy that someone at MS went into the level of detail they did here, even recommending and suggesting the use of gptgen and other 3rd party software. The solution had worked for at least 2 people so I figured why not give it a shot?

Some slightly hair raising command prompt from rescue disk commands later my original Win10 was booting again! Hooray! I've had no other problems but I am still slightly unstable so will have to look into that RMA I guess.

So if you want to convert from MBR to GPT my suggestion is : don't do it just for boot times, don't do it unless you have no choice, which may come with future OSes. If you need to do it, print out the link above or read it on another device and follow the instructions carefully and fully. You should be ok then.

Thanks for reading Hexus.

‘Stickied’ for future reference.

Very useful, sadly not as simple as changing from IDE/AHCI/RAID which I've done a lot of recently.
I will hang on to this as will no doubt come across this problem at some stage. I personally would just take the hit of only doing it when I do a re-install.

It's a bit scary when it BSODs, especially as you have passed the point of no return (bar a full restore) at that point. Although after doing this on numerous machines (both physical and virtual) I have yet to have the procedure go wrong or lose data.

While the recovery partition does not work, I did find that after deleting it and making a bit of extra space, once a new Windows 10 build is installed/upgraded the recovery partition can get recreated by the upgrade.

There is a Microsoft tool to do the MBR to GPT conversion. Has anyone used it? https://docs.microsoft.com/en-us/win...ent/mbr-to-gpt