Two main causes:
1. Shoddy disk
2. Shoddy controller
#1 can't be worked around, but #2 sometimes can:
https://doc.pfsense.org/index.php/Boot_Troubleshooting#.22Fake.22_RAID_cards_with_a_GRAID_error
Wiping the disk isn't a bad idea, quickest way is to let pfSense boot all the way up to the menu (may have to assign NICs first), drop to a shell and run:
dd if=/dev/zero of=/dev/ada0 bs=1M count=1
A little more than needed, but quick and sure to get the job done.
Also if your BIOS has any tweaks for the ATA controller, try toggling things like Legacy mode or AHCI, and a BIOS update isn't a bad idea. If there is a crappy soft RAID controller in the BIOS, disable it or set it for JBOD mode.