Same image, same PC, but different behavior on different boot medium?

I used method 1 from this set of instructions to install the current current version of HAOS.

I installed it on an old Great Wall small form factor PC with a cloned American Megatrends (2016) bios with UEFI 2.1. The exact bios version that it’s cloned from is unclear, but according to the website no more recent firmware is available for this motherboard.

EUFI is turned on, and the motherboard simply doesn’t have secure boot as it lacks the hardware module needed.

I followed the exact same instructions with an SSD and an SD card. I booted using a UBS stick with Ubuntu on it and used the Disks app to install the latest version of HA)S from the link provided above.

The SD card boots perfectly every time, no errors, no warnings, no problems other than it being a slow.

When I try to boot from the SSD it brings up a generic message saying that there is no bootable OS, press enter to continue. No logs, no other information.

I’ve tried setting each storage device as the master device for booting, and I’ve tried doing a manual override from the BIOS, but I get the same thing whatever combination I try.

I’ve even tried doing a cold boot each time from power down rather than a restart from within the BIOS. Despite everything that I do what happens is constant.

Does anyone have any idea why I can boot to the SD card but not the SSD, they should be absolutely identical, apart from one being larger and faster than the other.