unable to boot from USB

[khmiller]

2011 HP Pavilion p7-1080t CTO, OEM W7Home 64bit, i5, Pegatron MoBo "2AC2" 1.02 = HP "IPISB-CU" = "Carmel 2", BIOS: original AMI 7.03 installed 03/29/2011.

W7 > W8.1Pro upgrade in place wo problems 1/11/2020. Boot Mode: BIOS (Secure Boot not supported). HDD w typical MBR 3 partition System / C: OS / HP Recovery. Working flawlessly since shipping and after W8.1 upgrade.

Goal:

To upgrade to W10 now that 8.1 support is at bitter end of EOL while also installing a SSD for boot/OS/frequently accessed programs and directing Data (documents / pics / etc) to HDD as a second drive.

Dilemmas:

1. MBR or GPT? I'm still not entirely sure it makes a difference given how I use this computer - email, search, photo storage, documents, etc. no gaming, no add-on GPU, no processor-intense work, etc. TBH, I still don't really understand how each partiton style interacts with the Booting process: Legacy BIOS vs UEFI. I'm a home-tinkerer who hates throwing out old stuff if it still works, so I keep on rejuvenating stuff.

2. I cloned the whole HDD to a SeaGate USB HDD (same size capacity) using Macrium Reflect. This cloning overwrote the GPT format on the USB HDD that was already there from a previous clone of another unrelated 8.1Pro system. This previous GPT-formatted Clone booted and ran as an OS via USB on my HP Pavilion p7 and thus confirmed that I had a good clone. BUT - - the new Clone of the present p7 HDD, automatically done in MBR format by Macrium Reflect (bc ... it is a clone of the Pavilion MBR formatted HDD), will not boot on the p7. Curiously, it does boot and run on an extra laptop that is running W10.

* Boot sources on the extra laptop, a 2010 Dell Studio 1558 running W10 upgraded from OEM W7 and original BIOS A04, show a UEFI USB option - and thus the cloned USB HDD boots and runs.

3. The Pavilion p7 BIOS shows: "Boot Order" > "UEFI Boot Sources" > "Windows Boot Manager". There is no USB source option. The "Legacy Boot Sources" options likewise do not show any USB option / device, either with or without the external USB HDD Clone attached before powering on the system. I do not find any discrete "On/Off" toggle for UEFI mode anywhare in the AMI 7.03 BIOS options, nor is Secure Boot even offered as an option - so it therefore can't be Enabled I assume.

Solutions?

I have not tried to flash the p7 BIOS with any "update". The system is so old that HP Support refuses to identify it, other than in a manual search of a list "No Longer Sopported" devices. I did find much discussion of BIOS limitations on this MoBo, especially including limitations on which GPU's could be installed. These discussions point to the fundamental problem being that AMI / HP did not fully implement UEFI, or did so in an early 'evolving standards' kind of way. I have found HP AMI BIOS v7.16 Rev A (sp56122.exe) that supports chipset Intel H61, supposedly for W7 but apparently people have done it for W8. It is unclear if my MoBo supports this BIOS version, bc, well, HP no longer lists any drivers or support for the p7-1080. It's an Orphan.

I hesitate to attempt W8.1 > W10 upgrade when I'm not entirely confidant that I can recover with the Clone from a possible crash (bc I can't boot from USB). I've had an irretrievable BSOD happen once before during an Update and recovery from an system Image was no fun at all.

So: am I just out of luck? Should I attempt BIOS update? Would that even help? Do I give up on a GPT format for install of W10 on this MoBo / BIOS. I kind of expect to do an in-place W8.1 > W10 upgrade (thereby being able to use the existing authenticated Windows License), confirm all is functioning properly, then Image the system, then do a complete fresh install of W10 22H2 on an SSD, then repopulate a clean HDD w Data.

TL;DR - (after the 3rd paragraph): I can't boot from USB. Any thoughts appreciated.