Unable to disable UEFI

The USB drive works, I am currently reinstalling Windows 10 on a regular hard drive using my HP as I type this post, and I booted from the USB drive to test an LTSB version without an SSD.

I am aware of the installation, but I haven't found any other solution. I can't boot from a USB stick or from the Windows 10 CD with the Acer; UEFI blocks the boot as well as my SSD with Windows on it, despite having disabled secure boot. However, I can't disable UEFI to switch to legacy mode. The option remains grayed out no matter what I do.

For the USB stick, I use Rufus, but I also have another one with the base ISO, and both work since I installed the OS on the old computer.

Hello,
First, check that your SSD is properly detected in the UEFI.

It's most likely that your boot USB drive is not set up correctly, and/or the UEFI is not configured properly to boot from it.

And be careful not to confuse BIOS and UEFI, they are two different things. PCs have either a BIOS or a UEFI, with UEFI being the successor to BIOS. The two do not coexist, but it's common to refer to a UEFI as a BIOS due to linguistic abuse.

Most UEFIs allow operation like a traditional BIOS, enabling backward compatibility for OSes not intended for UEFI... However, if the PC's motherboard comes with a UEFI, it's better to keep it in UEFI mode unless there's a justified reason for using a traditional BIOS.

How did you create your boot USB drive? What is its name detected in the UEFI?

EDIT: installing an OS on one PC and then transferring the HDD where the OS was installed to another PC is not necessarily a good idea. If the hardware foundation of the original PC is too different from the destination PC, it can lead to hardware instabilities due to incompatible drivers, etc...

Ok, it wasn't that, the disk is indeed in GPT (I just checked with EaseUS partition etc... and I also tested it on another laptop).

So how can we ensure that it is recognized by the UEFI which seems to want the boot partition to start?

I have done quite a bit of research but I haven't found anything on creating an OS with a hard drive that includes a recovery partition like we can see in recent laptops.

Thank you, I will perform this operation after work, I will stop by to say whether it worked or not.

Hello,
Your SSD that contains Windows 10 must be in MBR; you need to convert it to GPT for your UEFI BIOS to be able to boot Windows 10 https://www.malekal.com/convertir-disque-mbr-en-gpt-sans-reinstaller-windows/

bazfile
Moderator/Security Contributor.
A hello, a response, a thank you are always appreciated.