In reply to Nick Brown - UKC:
Depending on how technical you are/how hands on you're prepared to be it might be worth looking at something based around Amazons S3 Glacier Deep Archive service.
This is designed for stuff you don't want to lose but in all likelihood will never actually need to retrieve, so it's crazy cheap for long term data archival. 4TB would cost you about US$7.38/month. You can have a petabyte for US$1100/month! You do have to pay for the data in/out though and it's not intended to be cheap for 'live' data.
It comes with a 99.999999999% durability rating. That's durability as in 'we won't lose it' rather than availability, which is 'you'll be able to get to it any time you choose', and the service has a 12 hour access time - basically it's probably on tape somewhere (or in reality, multiple somewheres - hence the durability) and a job has to be scheduled to go and fetch those tapes and copy your data back to where you can get it.
The advantage that this has over the NAS route is that at any sensible price point a NAS only has so many disk bays, so sooner or later you're going to max it out and then you're looking at replacing otherwise fine disk drives just so you can replace them with a larger device in the same bay. Plus a NAS is another piece of technology that has a small but ongoing requirement for your time in terms of maintenance.
Edit: Just realised I priced that up in the EU through force of habit. If you have no data protection requirement to keep your data out of the US then you can cut nearly 50% off your storage cost by putting it over there.
Post edited at 13:37