I guess it'd depend a lot on a few factors:
- How technical are you? (i.e. do you want to build / manage a box or do you want a solution which just works)
- How often do you need to access the older images?
- How much data can you bear to lose
Assuming that image loss is a big no no, then I'd be inclined to recommend using RAID (for me, mirroring is simplest although does come at the cosst of 50% of your storage space). This does also have the net effect of saying that a 4 bay NAS with 3TB discs actually isn't going to be suitable, which then leads to either:
a. A 6 / 8 bay NAS (or one could argue to do a low power PC build for this and run it as a server if you're happy setting up another box to do so)
b. Look at something like MS Azure to do so. If you're comfortable with using their BLOB storage and cold access, then you could be paying ~£45 a month to store 6TB in the cloud, https://azure.microsoft.com/en-gb/pricing/details/storage/blobs/ ). If you've got a website which sells images directly from the image store then this also has the nice effect of making everything available to that site (assuming you've coded it that way of course )
c. Run a micro server (HP are forever doing sales on their ones - http://www.ebuyer.com/722189-hpe-proliant-gen8-4gb-ram-microserver-ebuyer-com-819185-421 - currently £122 but without OS or discs) and then fill it full of RED Pro discs ( http://www.ebuyer.com/742196-wd-red-pro-6tb-3-5-sata-nas-hard-drive-wd6002ffwx )
Disc wise, I'm very happy with my WD Red Pro, they're designed for tasks like this. The warning that I was once given (and I'm not sure how true this holds today) is that standard consumer discs aren't necessarily ideal for running in a RAID configuration due to the way that they're designed to self correct. Whether this is still true or not, I don't know sorry.
Hope this is useful
If you go for a NAS, you'll still need a DAS to create backups anyway. NEVER assume mirroring on a NAS is a substitute for a backup.
I personally wouldn't fret too much about a futureproof NAS, especially with an 8-bay one as it'll be hideously expensive and even a 4-bay one could theoretically be overkill for your needs (12TB drives now exist, albeit are very expensive). I'd personally stick to a 4-bay one and buy large drives for it (8TB ones are 'affordable' in the scheme of things) and worry about a NAS with more slots as & when the time comes.
As for brands, I've got a two-bay Netgear one... it does the job but is a bit slow. Work's got a couple of 4-bay Qnap ones and they're incredibly fast, and have a really nice management interface to them.
@RedChecker, that's a good point - redundancy is not the same as a back up.
Ideally, you'd have some other storage in addition to the primary (whether this be cloud or a USB enclosure with a large disc(s) would depend on the cost / benefit etc. Given the volume of data that you're talking about, burning to optical discs is probably going to be impractical hence the Red's suggestion of another store (be it DAS or cloud or tape drive etc. but these can add up quite quickly if you're not careful) is the gold standard.
FWIW I use RAID1 at home for most of my data and then use cloud storage (Azure) for irreplaceable things / important docs
It's going to depend on your personal set up, a DAS is not so effective for me as I'm working off a couple of Mac's and the ability to share is priceless. Definitely consider a RAID system, if you go RAID 5, you're not giving up 50% of your storage, but one drive, so 4 x 4TB will give you 12TB of storage with 4TB reserved for fall over. Check out the Synology RAID calculator and you can play around with different scenarios. https://www.synology.com/en-us/support/RAID_calculator
Cost is going to be another factor, you're paying more for a NAS, but in terms of flexibility, they do so much more than a DAS. Budget permitting I'd go with Synology or QNAP