I've tried 2 other USB devices, as well as using all of the USB ports available.
I reflashed another USB drive and "progressed" so that it now lastly says:
da0: quirks=0x2<NO_6_BYTE>
Trying to mount root from ufs:/dev/ufs/OPNsense_Install [ro,noatime]...
I'm using an ASRock 970 Extreme3 mobo with latest BIOS & a Phenom 975 AMD CPU, as well as an NVIDIA GTS 450.
Did you find the issue?
I am using the ASRock 870 Extreme 3 and having the same issue. I also tried via DVD and after /BOOT/LOADER... Found it spins for HOURS until showing a pipe | on the next line.
Since you're both running legacy AMD CPUs, it's likely related to a FreeBSD12 installer bug + the PTI issue. More info in this thread here with a work around at the bottom: https://forum.opnsense.org/index.php?topic=23348.0
That thread was indeed the answer! vm.pmap.pti