TS431 Replacement Drives DOA?

[no1cartman]

Hello
I have TS431 running QTS 4.3.6.2050. 4 bays,
Disk 1 is a 2TB Seagate Barracuda and runs the system software and basic storage
Disk 2, 3 Are 4TB Seagate Ironwolf NAS drives running in RAID 1
Disk 4 is 4TB Seagate Ironwolf NAS drive over flow storage.

Disks 2,3 were running out of space so brought 8TB ironwolf drives to replace these. I followed the process in the storage manger and where you replace 1 drive at a time and let the RAID rebuild. I had done this process before moving from 2TB to 4TB drives except this time things haven't gone to plan. I replaced disk 2 and it rebuilt the RAID and it said was successful so I went to replace disk 3 and it wouldn't connect to the drive and gave errors like disk 3 failed, failed to access the installed drive, check the drive for errors. So i put the disk in a caddy and connected to my pc and ran seatools tests on the drive and they all passed so I tried to get the NAS to pick up the drive but same result. Then also in the log files disk 2 was now having S.M.A.R.T errors 197,198 sector counts. And as expected getting errors now the RAID is degraded.
So questions
Could both my new 8TB drives be DOA? they are still under warranty so do I send both back? or 1 at a time to rebuild the RAID
As I had already started the process of replacing the drives can I stick the old 4TB drive back in with out losing the data.
Log files and SMART error attached

[dolbyman]

A very important and often too late lesson to learn...ALWAYS have external backups..even more so during risky maneuvers like raid rebuild/expansion

Either the backplane or the new disks are shot...you might be lucky and only the new disks are shoot...you can pop one (!) of the old disks back in (not both as they are desynced with each other) and see if the volume comes back up degraded

backups should be top priority after

[no1cartman]

I use the NAS as my Backups from my other devices

[dolbyman]

Then the data is still there on your originals..and all data can be recreated from there (if it's not on the original sources anymore..then the NAS is NOT a backup)