"Asking around among our tech-savvy friends though, no one has a good answer to the question, 'how would you backup 20TB of data?'. It's not like you could just plug in an external drive, " tells me that you have NO "tech-savvy friends". None. Zip.
Right now, I'm on my biweekly offline backup - that's where we rsync from the online backups to offline backups. This is the 10 3TB drive, if you're interested, out of 13.
Now, if you actually had any "tech-savvy friends", as opposed to people who think they're "power users", they'd have pointed out, first, that what your tech-savvy-friendless friend had was *not* a 20TB file, but many, many files. It's certiainly not any kind of problem to partition them - y'know, divvy up the RAID and have movies and music subdirectories, and break that up by moving all the movies whose title starts with "A" under /movies/A.... and then rsync (or however you prefer) copy enough to close to fill one drive, then swap drives....
Oh, and why can't you do it in an external drive? Certainly, that's what I'm doing *right* *now* as I type with those 13 3TB drives.