Restoring system images on GPT systems  

I have secure boot enabled (has been since day one), I have been able to boot from USB, both Win10 and Macrium with no issues, can even wipe drive partitions and clean install Win10 without having to turn off secure boot.

You're both correct, if the flash drive is FAT32, and has a TPM (Trusted Platform Module) approved UEFI Bootloader on the thumb. That's why Macrium will work and Windows created will work. They take care of that. Others are stopped dead. Sometimes it's hit and miss, used to be very miss. That's why in almost all the tutorials here involving booting from Thumb/DVD, it states "You may need to disable Secure Boot first".

Depending on how the Thumb is created affects the outcome.

There are a few versions of the TPM, also different implementations of secure boot. The main thing to remember is to TEST any bootable media you create from any application/ OS to make sure you can boot your computer.

Fixer said:

Reason I have not been responding is I think I may have bricked the laptop. I needed to run a utility located on a flash so disabled secure boot, but I also needed to change another UEFI startup setting to Legacy only. Since then I have not been able to access bios/uefi to undo the changes and only thing I am able to do is see the flash menu and run its utilities, including Partition Wizard. That sees the hdd partitions but most changes are greyed out. Without reverting the changes I made I cannot boot from the GPT hard disk. I feel there must be a way to regain access to bios/uefi,. normally by pressing Enter at boot, but right now I'm stuck.

Morning, Fixer. That's an odd combination..... flash utility menu, and Partition Wizard. I'm "assuming" you mean MiniTool is Partition Wizard. That's a Windows app, not anything to do with BIOS anywhere.

Anyhow, can you pop the CMOS battery? If so, pop it and wait a minute or so. Put it back in and power up. If lucky, it will reset the BIOS firmware back to Factory.

Fixer said:

Thanks for that. I was able to disconnect the cmos battery for a few minutes but it hasn't reset itself. I only mentioned Partition Wizard because I wanted to see what it was saying about the currently inaccessible GPT hard disk. It could see it and its partitions but I couldn't see any way it might be of help, but I do have a plan of sorts. I do have a spare MBR hdd and using MR from the flash disk I can probably restore to it the MR image I took prior to the conversion to GPT. That should get Win 10 up and running again and I'll have to reassess the situation at that point. I'll keep you informed if that's ok.

At your leisure! Looks like you have your hands full for a while. Post back how it's going when you can, or if any questions, or you just feel like screaming.

Fixer said:

OK, plan has worked, the laptop is working on the spare MBR hdd after restoring the image I took before converting to GPT. I'm going to have to try reflashing the BIOS as I still can't access that and I need to find out what I need to do to be able to use the GPT disk without causing another problem.

Thanks to everyone for your interest and contributions which I shall be looking at more closely now the main issue has been resolved

Glad to see you're at least back up and running. De-bricked, as it were.