That's why there is RAID 6
Not looked into RAID 6*, used to have a 7-disk SAS RAID5 setup though - blazing fast, a pretty lightshow (as each drive had an activity LED), but damn noisy: not something you want in your bedroom.
* Oooh, field theory. I'll bookmark that and stretch my maths muscles with an in-depth read one night.
You're assuming people have a friend they are prepared to share their pr0n collection with
Kryptos
Seriously though. I don't have unlimited bandwidth, and my upload is extremely slow. I have over 1000 movies and 50+ TV shows with multiples of seasons each. I'd hate to think of re-downloading all of that. That's why it's on a Synology NAS with RAID 5. Yes, a double disk failure would be the end of that, but the odds of a double disk failure occurring are far less than a single disk failure.
True, but the chances of a single disk failure + block errors on a remaining disk are much higher.
My brother is in a similar position - approx 20TB of (mainly video) data, with no obvious way of backing it up regularly. We've looked at possible RAID solutions for disk redundancy, but most are cost prohibitive compared to the benefit (ie not protecting against fire/burglary/multiple disk failure due to power surge).
In his case, he probably has less than 10GB of irreplaceable data - the rest is multimedia. Taking periodic backups (of the media files) offsite is probably the only feasible solution in his case, meaning a complete duplication of storage capacity.
Sigh. I remember when I has a 80MB IBM 2.5" hard disk in my A1200, and wondered what I'd do with all that space.