It's an 8GB USB drive you're booting from yes?
Try selecting option 3, boot from USB, at the initial boot menu. That introduces a delay so that the usb device can initialise before the kernel tries to mount it. If that works then you can make that change permanently by adding:
kern.cam.boot_delay="10000"
to the file /boot/loader.conf.local
That file will have to be created. See:
https://doc.pfsense.org/index.php/Boot_Troubleshooting#Booting_from_USB
You can do it in one easy step by executing the following line at the Diagnostics: Command Prompt: Command: box
echo 'kern.cam.boot_delay="10000"' >> /boot/loader.conf.local
Steve