ReadyNAS 314 - a year of hell and now lost 10years of data!

Disks can and do fail at any time, sometimes without any warning. However in this case you had plenty of warning you had disk troubles before the disks failed but chose to neither make a backup (just in case the errors indicated the disks were failing) nor replace the disks nor ask on the forums what the errors meant. Errors coming immediately after an update is not unexpected as the disks are routinely checked on boot (e.g. when doing an upgrade, but could reboot etc. the NAS for a range of reasons) and daily in the early hours of the morning, I believe. If in doubt about the health of the disks you could have checked them using e.g. SeaTools (for Seagate disks) or WD LifeGuard Diagnostics for WD disks. It is possible to check the disks in the 314 using those tools while the disks are still in the ReadyNAS 314. I've checked a SeaGate disk in a 312 before using SeaTools.

You could try cloning the disks using ddrescue or something like that onto good new disks, but it could be too late for that to work/help.

A number of ReadyNAS Users use their ReadyNAS for primary storage and e.g. Crashplan to backup the ReadyNAS to the cloud.

RAID is great, but it is a first line of defence, multiple disk failures, accidental file deletions and other problems can happen. I would recommend using RAID-6 or dual-redundancy which greatly reduces (but does not eliminate) the risk of losing data due to multiple disk failures.