Converting mbr to gpt failed for Win10 laptop

I have an old Dell Latitude laptop that a relative gave me, rather than thowing it out, and it was running Windows 7. So, I upgraded it to Windows 10 -- a couple of years ago -- and it's been working fine since. It's now running 19045.3636 Pro.

But only recently, I was scrolling through its BIOS settings and discovered that it can boot using UEFI, so I set about to convert my MBR, legacy boot Win10 to GPT, UEFI boot. I read this was a simple process using mbr2gpt.

I ran mbr2gpt validate -- and got no errors, so I ran mbr2gpt convert -- and it worked OK (conversion completed successfully), but it ended with the following message:
Failed to update ReAgent.xml, please try to manually disable and enable WinRE.

So I ran reagentc /disable -- and got:
Windows RE is already disabled.

So, I rean reagentc /enable -- and got:
Unable to update boot configuration data.

When I reboot the Dell, I get the error:
No bootable devices

I have a desktop that runs both Win 11 and Win 10, and with a dock, I can access the laptop's SSD to do any file changes.

Yeah, I know I could clear the drive and reinstall Win10 from scratch -- but I am trying to get this conversion to work without having to resort to that.

So, what can I do to repair the boot?

Have a look at the disk and check there is an esp partition fat32 format . Then fix the boot critical files using bcdboot.

SIW2 said:

Have a look at the disk and check there is an esp partition fat32 format . Then fix the boot critical files using bcdboot.

Confirmed the esp was there but when I restarted the PC, it did not give me a troubleshoot option, only shutdown.

I'm going to restore the drive from backup and use Minitool Partition Wizard to do the gpt conversion -- as their website claims this will work when mbr2gpt fails.

I would do it from booted media with cmd prompt. Or from another os ( same version or more recent ) if you are multibooting

bcdboot windowspartitionletter:\windows
e.g.
bcdboot d:\windows

if there is only one disk attached that should find the esp partition automatically.

if there is more than one disk you can specify the target esp partition with the /s switch

bcdboot windowspartitionletter:\windows /s esppartitionletter:
e.g.
bcdboot d:\windows /s z:

or

bcdboot windowspartitionletter:\windows /s espdevicepath
e.g.
bcdboot d:\windows /s \Device\HarddiskVolume1

I have used Minitool Partition Wizard once and did not have any issues.

The op has already converted the disk to gpt. Matter of seconds to run bcdboot command.

SIW2 said:

I would do it from booted media with cmd prompt. Or from another os ( same version or more recent ) if you are multibooting

bcdboot windowspartitionletter:\windows
e.g.
bcdboot d:\windows

if there is only one disk attached that should find the esp partition automatically.

if there is more than one disk you can specify the target esp partition with the /s switch

bcdboot windowspartitionletter:\windows /s esppartitionletter:
e.g.
bcdboot d:\windows /s z:

or

bcdboot windowspartitionletter:\windows /s espdevicepath
e.g.
bcdboot d:\windows /s \Device\HarddiskVolume1

Thanks for all the details -- but I just stumbled upon this thread here -- Conversion of Legacy to UEFI Win 10 -- and am reading through the comments. In it, you suggested using Disk Genius, so I have downloaded the Free version and will give it a try.

I just have to ask..... after you converted the disk from MBR to GPT did you change the BIOS setting to boot in UEFI mode? I don't see any indication that you actually changed the BIOS to boot in UEFI mode - only that you noticed the option was there.

Mark Phelps said:

Thanks for all the details -- but I just stumbled upon this thread here -- Conversion of Legacy to UEFI Win 10 -- and am reading through the comments. In it, you suggested using Disk Genius, so I have downloaded the Free version and will give it a try.

diskgenius will convert an mbr disk to gpt or vice versa. It doesnt automatically create the esp partition. After conversion rt click unallocated space and select create esp/msr. Click Save All and let it format the esp. ( The old 100mb system partition could be deleted to create the aforementioned unallocated space ). Then populate it with bcdboot command.

Sounds like a lot of steps if you havent done it before, but when you have done it once, it all makes sense. Takes about 60 seconds to do the whole job.

This is why I prefer to make a backup on a drive that is equal or greater then the one I have to avoid things like this from happening, when I play with.......