After successful installation, I got the following error when rebooted:
Mounting from ufs:/dev/da1p2 failed with error 19.
Loader variables:
vfs.root.mountfrom=ufs:/dev/da1p2
Manual root filesystem specification:
<fstype>:<device> [options]
Mount <device> using filesystem <fstype>
and with the specified (optional) option list.
eg. ufs:/dev/da0s1a
zfs:zroot/ROOT/default
cd9660:/dev/cd0 ro
(which is equivalent to: mount -t cd9660 -o ro /dev/cd0 /)
? List valid disk boot devices
. Yield 1 second (for background tasks)
<empty line> Abort manual input
I typed in ? and press enter, the following (among many others) are shown:
ada0p2
ada0p1
ada0
da0p2
da0p1
da0
I typed in: ufs:/dev/da0p2 and press enter and it started up.
It would be nice if this is fixed so I don't have to do this. I am not sure if adding to /boot/loader.conf would work, but I can give it a try.
vfs.root.mountfrom="ufs:/dev/da0p2"
This does not happen with PfSense, but I would like to move my devices to OPNSense.
I can confirm that once I log in and get into Shell, appending the /boot/loader.conf file as mentioned above works when restarted. This is mentioned here.
https://forums.freebsd.org/threads/booting-from-usb-error-19.57429/#post-327585
So, I stumbled on the first boot after installation, and using "?" command to find out various partitions, and using it to manually enter: ufs:/dev/<partition name>
helps moving forward.