This is the first time I've ever had a problem with the USB enclosures. And its fantastically rare, roughly one corrupt 512b block per TB of data written. With a btrfs-raid1 it's self-correcting on reads, if I didn't look at dmesg I'd never know.

I've figured out it only happens if I'm streaming data over the NIC at the same time as writing to the disks (while copying from one local volume to another), but that's all I really know right now. I seriously doubt it's a software bug.