Failed password for invalid user ftpuser from 13.94.255.171 port 53232 ssh2
Are you directly forwarding the port for FTP and/or have the server in your router's DMZ? Not the greatest idea as eventually a brute force attack (which is currently happening) will eventually succeed. Use a VPN or reverse proxy or the like.
By and large drive failures are exceedingly rare
This
Nov 12 16:08:23 unRaid-DINO kernel: ata3.00: failed command: READ DMA EXT
implies that you've got poor cabling (loose sata and/or power -> SATA connections are (pardon my French) shit, especially if you ever add in splitters etc into the equation) Now, seeing as how the drive in question is returning no SMART data at all it is either
The above problem (that code snip however was from your parity drive)
The drive actually is completely dead
#1 is far far more likely. Reseat all the cabling (both ends), and after you've done one drive, make sure you don't touch it (and hold your breath) when doing the next drive. Additionally, if you problems seem to be always on Disk 2, then do NOT use locking SATA cables. Most of them actually do not meet spec on the requirements and hold worse on WD drives than the non-locking type (which by definition ARE spec)