I've lost 3 drives of the R6 in the past 3 months. It's crazy as they all had ~9,800 unparked hours... they would have made terrible system disks.
When the second failed, I replaced it, marked it unconfigured, then started rebuilding the RAID 5 array. Before it completed, a third drive failed. I replaced it with a new unconfigured drive, but of course, I cannot bring the array online to affect a rebuild.
The manual says to contact support if this happens as there _may_ be data loss. I do not have a backup as I thought what are the odds of losing 2 drives. If only I knew they were Seagates...
I contacted support, sent system history files, but to go further, I have to pay. It seems like this is something that can be in the manual or have double "are you sure?" in the interface so the end-user knows they _may_ lose the data. I shouldn't have to pay $80 to get told the way to force it online to rebuild. It's not like you're diagnosing cable or clean power issues (n.b., it has been on UPS conditioned power its whole life).
If I can't find the way to force a rebuild (which _may_ lose data), I'll honestly just reformat it all and look for replacement hardware before the fourth drive fails.