Assuming you do mean NAS, not HDD, the Netgear ReadyNAS range offers great value - search Amazon etc.
You can buy either populated devices (complete with HDD installed in them) or you can buy diskless models and put your own HDDs in them (this is usually cheaper). The diskless models allow you to select the make and size of HDD that works for you. Just one word of caution with diskless NASs - ensure the HDDs you select are compatible with your chosen NAS (Netgear web site has a page of compatible HDDs for each model of NAS).
Still confused ?... don't faff around, just get a Unitiserve
you know it makes sense !