Agreed on 4TB to be a reasonable upper limit for RAID1 if URE rate is 1e-14. However, why would 1 in 20 rebuilds fail? And if it did, why would there be a data

Agreed on 4TB to be a reasonable upper limit for RAID1 if URE rate is 1e-14. However, why would 1 in 20 rebuilds fail? And if it did, why would there be a data loss? The data is OK as long as the remaining disk is alive, you can retry the rebuild.