There is a device that Maplin Sell that does NAS + USB and comes without
a HDD. It uses a normal PATA drive.
Most (but not all) NAS appliances seem to come with disks already
inserted. However there are a few that are using SATA drives in Caddies
where you can but just the chassis and bare caddies. I think there
might have been some advertised in a recent edition of MacWorld.
