Even if 100% of that 20TB is legally owned content, recovery is a huge process: re-ripping hard media is still awfully slow -- if you can even find where you stashed it (I think a few CD's have walked off the reservation)
Purchased digital media is no better: you've got many sources to find it from, and it may disappear: preview tracks, live tracks, etc. may disappear when they stop updating their MySpace, or a local distributor goes belly-up. That's also assuming you're still using the same providers: if you had download privs on some of the music servers of the 2000's, you'd have 'ownership' of that media, but you may not be able to get it again if you aren't still paying for the account.
The most economical and reliable is probably a mirror RAID array. It sounds like this guy accidentally issued a command to erase the content, rather than a RAID failure. Ordinarily, the RAID should be good for most stupidities, but this falls a little outside that. The question is, if you have mirroring software, how frequently does it try to match, and would it clean off the mirror too?