@stephenw10 It boots ok when I move the flash drive back to USB 2.0 (although it is slow, takes 2 minutes 10 secs), but I guess it's not the worst case if it won't boot on USB 3.0.
It comes up with these errors in the Filter Reload log:
There were error(s) loading the rules: pfstl: DIOCBEGINADDRS: Operation not supported - The line in question reads [0]
Not sure if this is anything to worry about, comes up every reboot in the GUI.
Thanks again for all of your help!