might have to do with installing
in-the-background
geom_bsd.ko, geom_mbr.ko, geom_label.ko
before sysinstall detects the drive devices.
(I came across that issue from v7 > v8, never
installed from sysinstall v8, so I do not know
the precise fix. Other posts mention those three
ko's however and a slight chance someone posted the
syntax or a workaround. )