There are hundreds of off the shelf solutions for this. Most if not all multi-HDD external Drives allow for mirroring of the data. (RAID 1) One problem with them is you have two identical drives in a single enclosure that is usually doing some sort of proprietary data handling. Too many opportunities for what still ends up as data loss if one of the drives dies or the drive controller craps out. The other is that you are only getting half the storage you are paying for.
The Drobo has become a popular choice for many but there seem to be issues that are not fully addressed. It is one of the few solutions that can grow as your needs for storage do.
After hunting, reading and spending way too much money I have decided that RAID is not the way to go for me and my needs for simple data mirroring. I have two size and speed matched external drives connected via Firewire 800 to my media server. On the server I run Chronosync with a scheduled job to do a full sync between the drives every 3 days. Sounds a bit overwrought but it meets my needs of data protection, network access to the data and no need for my intervention. This is a Mac only product but I am sure there are Windows products available.
As for the NAS question, it does allow connection to the data via your network. Most NAS equipped drives do not like trying to have a drive mapped via network and USB at the same time on a single machine. Remember though that NAS often adds some sort of overhead to the connection.