Shoot. I was hoping that was it. I haven’t been able to reproduce this issue. The drive I was backing up to appears healthy. No reallocated sectors, no I/O errors, no UDMA errors.
It’s not my primary backup, so I’ve fully overwritten the backup several times (~15.5TB), using rsync with --checksum on a second pass for verification. Can’t get it to do anything out of the ordinary. So I don’t know what it was… 
