Tried some more stuff:
- Checked motherboard firmware. It’s the latest at 3.2.
- Turned off “fastboot” in UEFI. No change.
- Checked motherboard battery battery at 2.96 vdc
- Checked SATA configuration settings in UEFI under South Bridge Chipset Configuration. All looks nominal.
- Tried different settings for UEFI CSM settings. No change.
- Tried different settings for UEFI PCI ROM Priority. No change.
- Reloaded the OS from USB stick using “fresh install” option this time selecting BIOS instead of UEFI for “Boot Mode” and THIS WAS IT.
Now the system boots with no issues or errors. I don’t understand what exactly this means as the motherboard is UEFI but I guess I was wrong to choose “Boot via UEFI”. Sheesh.
Anyway, maybe this long, painful saga will help someone else in the future.
Thanks to those who offered their time and assistance.
