Ddrescue

make install clean -C /usr/ports/sysutils/ddrescue/

ddrescue /dev/ad0s1a /dev/ad6s1a

 Press Ctrl-C to interrupt rescued:    9725 MB,  errsize:     512 B,  current rate:    2291 kB/s ipos:    8094 MB,   errors:       1,    average rate:   31372 kB/s opos:    8094 MB 