But for a "dedicated" NAS box that can hold up to 6 disks, an A20 would be a rather odd choice.
its SATA interface is also very very specifically a single LUN.
AFAIK it is compatible with sata multipliers (see discussion at http://linux-sunxi.org/SATA), tho you have to request it explicitly with the "enable_pmp=1" module argument.
Stefan