Perfect, thanks Megahertz again for your help!

It's a Seagate backup drive connected via USB:
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All done, no active partitions on disk 2! Thanks!

Done! Thanks
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Sure:
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Looks like it?!
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Well we successfully have a Win11 install - thanks Megahertz and Pentagon for your help!

Windows update is behaving a bit oddly, in that it still thinks I should be installing the old Win RE...

Thanks - I will take a look at that!

- - - Updated - - -

Ok, so I'm going to put an ISO on a USB, mount it and install from there (let me know if that's not right).

PC Health Check app...

Ah! Yes, the whole point of this process is to enable UEFI so I can get to WIN11 - will the automatic upgrade (leaving all files in place) sort the RE issue?

Thanks both - I did wonder if that was a typo but didn't want to assume! Has error I think as Pentagon was predicting:
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Thanks Megahertz, really appreciate your help.

I'm running Win 10 Home. The bits in disk part worked fine, but last two instructions didn't - first got no response and second an error? Happy to...

Done!
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No problem! Success:
C:\WINDOWS\system32>bcdboot C:\Windows /s W: /f UEFI
Boot files successfully created.

Renamed successfully but last bit didn't work:
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Done! So we now have:
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Thanks - here you go:

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Thank you! That worked. Changed BIOS to UEFI and successfully rebooted into Windows! Amazing! Thanks to you and Pentagon for your help so far! Disk management is now as below:

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Ok - it won’t shut down via the power menu - goes to the blue screen with ‘shutting down’ on it, goes black, and then goes back in to Windows! Shall I pull the plug??!

Thanks Megahertz - just to check - do you mean boot from C after the BIOS change? Or boot from the MCT USB?

On drive 2? No - used that MCT tool straight to USB.

No idea! That's an external backup drive and has only ever been used as a backup!

Thanks Pentagon - no files copied?

C:\WINDOWS\system32>XCOPY R:\Recovery\WindowsRE C:\Windows\System32\Recovery
0 File(s) copied

I have Windows set to auto-update so not sure why that's so...

Thanks Pentagon! Steps 1 and 2 couldn't find it but step 3 has got those three files!

C:\WINDOWS\system32>dir R:\Recovery\WindowsRE /a /s
Volume in drive R has no label.
Volume Serial Number...

Thanks Pentagon - first bit gives an error - I thought maybe I should have done 'C' as that's my main drive, but same result:

C:\WINDOWS\system32>mountvol B: /s
The parameter is incorrect.
...

Thanks Pentagon for your reply, much appreciated. I got to the format line then get an error:

DISKPART> format quick fs=fat32 label="System"

Virtual Disk Service error:
Format is not allowed...