Doesn't that keep more than two copies? RAID 1 is expensive enough as is. I just need to be able to pop in a replacement drive if one fails so I don't lose data.
The chance of a rebuild taking longer than a failure occurring is non zero. RAID 1 is less protection that raid 50 or raid 60, plus depending on how many disks you have you will save space and get better performance.
I'll have to watch a video later I guess. I know it's possible to lose a drive while it's rebuilding, but it's improbable, and I figured it takes more space, I'm okay taking that risk since I'm not handling irreplaceable data, just my personal TV/movie archive for now. But if it can save space, I'll have to consider it.
It is far less improbable than you think, especially if all of your drives have similar age/wear - as would be the case if you bought all 40 around the same time.