Do you really need to back-up that much data?
I'm just speaking generally here, there are certainly cases where someone would need to back up this much data, but for your home media library? If we're talking movies, 20 TB is roughly 20,000 movies (for sake of argument, I'm not considering music). At what point is this just digital hoarding? I used to keep a large collection of movies, mostly pirated, and eventually realized that:
a) I was spending more time and money managing the collection then I wanted to.
b) That I rarely watched many of the items in my library.
c) That I was placing myself in legal jeopardy by storing so many illegal copies.
d) Anything I did want to re-watch I could get from Netflix, the public library, or download.
Music would be slightly different, as I could see where music is in some kind of constant rotation, but again, how much of it are you actively using? I'm just playing devil's advocate here, but I think this kind of collecting/hoarding is a byproduct of pre-internet scarcity.