So I might regret this, but I just picked up two of these WD 8TB external drives. I've ripped them open and one by one replaced two 3TB drives in my Synology. Once this second one is in place I should have much more space, and I plan on swapping out more in the future. I'm planning on ripping most of my old DVDs and storing them on there. These drives probably aren't very good, but they seem to be liked enough by other Synology owners who are just looking for inexpensive storage. I'm not trying to beat any speed records, I'm protected from one disk failure, and if the whole thing goes down I'm not storing anything vital on there (though what little I am is being backed up to the cloud).

I'll probably kick myself for not splurging on 10TB right now, but the cost was about %40 higher for that additional 2TB. Currently the 8TB drives are on sale for only about $15 more than the 6TB drives so that was a no-brainer.