I really did get a kick out of some of these responses. I sell data protection products for a living and 20TB is what I would consider an average small/medium customer. Every business these days has tens of terabytes of data. Of course they all need to backup their data, so there is nothing novel here. We have plenty of customers backing up hundreds of petabytes of data. Every dataset just needs a plan for backup, pretty simple.
The way I see it, this guy has a few options. One option is to just get more disk and make redundant a redundant copy. This would have have saved him in this case of the mistakenly erased raid, depending on how smart his sync script is. But a redundant copy is not a valid genuine backup plan. So many types of failures will show the holes of the dumb redundant copy.
The other option for a home user who's not looking to spend a bunch of money, is LTO6. They hold a sufficiently large amount of data, so only a handful of tapes will be needed. LTO6 drives are cheap enough, they won't break the bank. Since the data is on tape, you can shuttle the tapes to an off site location. Seems pretty simple.