BIOS update via USB is failing. USB is not bootable.

[BFG_]

Trying to update BIOS from here:

https://support.hp.com/gb-en/drivers/selfservice/swdetails/hp-pavilion-23-q100-all-in-one-desktop-pc...

The PC existing BIOS is v0.12. It will have windows 10 on it but currently there is a new blank SSD.

I'm using another PC to run the download sp97015.exe, which runs  "HP BIOS Update and Recovery" and choosing the option

"Create Recovery USB flash drive"

A fresh format on a (definitely 100% tested and working) 4Gb USB drive, 

"The recovery flash drive was created successfully" is the result.   It creates a FAT32 file, and the contents are browseable.

--------------

However. It is not a bootable USB. It's not recognised by the q199na either in EFI or Legacy mode.  I tried it in other PCs and no, not bootable.  This seems like a faulty BIOS update program.

[BFG_]

I found a reference that the USB stick needs to be a maximum of 1Gb, which is a ludicrously short-sighted design.

Anyway, luckily I still had a working 1Gb stick from about 10 years ago.

Tested the stick thoroughly, all good. repeated the creation process again, and the process said "The recovery flash drive was created successfully" 

The PC does not recognise the USB as bootable.

I tried stepping up one by one. So instead of going from v0.12 to v0.18, I tried v0.13 next. Same result.