Error during installation.

[wblock]

Boot from mfsBSD.
Log in as user root, password mfsroot.
kldload geom_raid
graid delete raid/r0

This assumes some things, but is likely to work.

 

[tvsjr]

If you boot to FN or to your choice of live Linux/BSD distribution, do you have direct access to the drives? If so, usually overwriting the first bit of the drive will be sufficient:
dd if=/dev/zero of=/dev/<your_device_name> bs=1M count=100

 

[wblock]

Oh, and https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/geom-graid.html#geom-graid-unexpected shows an extended approach.