On an appliance I reinstalled with pfSense I was facing a problem booting the system from an usb stick.
The system was not able to mount the partition correctly because it wasn't present at that time. For me this looks like a timing Problem. The system booted without a problem if I didn't connect the USB stick directly. I used an USB-hub while installation and booting from that USB stick while it was connected through this hub worked like a charm.
My Problem was similar to this. https://redmine.pfsense.org/issues/495
The fix was quite easy. I booted the system using the USB-hub and modified the loader.conf
I just had to add this at the end of the file:
and everything worked fine even if I connected the USB stick directly to the appliance.
Hope this helps you if you're facing a similar problem.