The key concept here is how valuable your time is to rebuild your collection. I have a ~92TB (8x16 radiz2) array with about 33TB of downloaded data that has never been backed up as it migrated from my original cluster of 250GB drives through to today. I think part of the key is to have a spare drive on hand and ready to go when you do lose a drive, to be swapped in as soon as a problem shows up, plus having email alerts when a drive goes down so you're aware right away.

To add a little more perspective to my setup (and nightmare fuel for some people), I have always made my clusters from used drives, generally off ebay but the current batch comes from Amazon's refurbished shop. Plus these drives all sit externally with cables from SAS cards. The good news is this year I finally built a 3D-printed rack to organize the drives, matched to some cheap backplane cards, so I have less chance of power issues. And power is key here, my own experience has shown that if you use a cheap desktop power supply for external drives, you WILL lose data. I now run a redundant PS from a server that puts out a lot more power than I need, and I haven't lost anything since those original 250GB drives, nor have I had any concerns while rebuilding a failed drive or two. At one point during my last upgrade I had 27 HDDs spun up at once so I have a lot of confidence in this setup with the now-reduced drive count.