A user can modify files nearly anywhere on the running CD via UnionFS , a filesystem that can stack a read-write filesystem ( in this case , a dynamic ramdisk ) on top of a read-only filesystem ( the CD media ) . A user can modify files nearly anywhere on the running CD via a union mount filesystem ( UnionFS or AUFS , depending on the Finnix release ) , stacking a read-write filesystem ( in this case , a dynamic ramdisk ) on top of a read-only filesystem ( the CD media ) .