Unable to boot with serial image

[chaft52]

Hi,

I've an old Applicance MB-7573-A, this appliance was used with the 16.7 version of OPNsense.

I try to install the last version, but with no success...

Now, i try to create a USB Flash Drive with the "serial image" of OPNsense, but the USB Flash Drive is not recognize as a bootable device.

Have you encounter this ? I'm sure it's a trivial things, but i don't see where i made the mistake

Do you know if it's possible to upgrade the bios of this appliance (MB-7573-A) ? I didn't find any ressources for that