I went into the BIOS and completely disabled USB (used PS/2 adapter for my usb keyboard) and everything worked

since I just want this as something to ssh into over the network , that will be OK

probably some quirk of the usb controller on that specific platform that FreeBSD can cope with and which we don't handle.

yes and probably wasn't tested just due to not being a popular platform

that advice was the clue I needed to figure it out

xhci is usb3, and yes, there is known issue about usb3 + boot disk.

illumos.org/issues/15502... altho this one does behave different.

→ BUG 15502: 'Device is gone' error when booting from USB3.0+ disks (New) | code.illumos.org/c/illumos-gate/+/2731