And also no reason to remove unless you intend to replace it with a different disk. Still trying to decide whether or not to believe in parity or not. I filtered out a lot of that.
So the scheduled parity check was configured to write corrections to parity. Did you let it complete?
Apparently you rebooted after that scheduled parity check
Oct 1 22:48:06 baconator kernel: Linux version 4.19.107-Unraid (root@38721b48cdfb) (gcc version 9.2.0 (GCC)) #1 SMP Sun Mar 8 14:34:03 CDT 2020
no indication of an unclean shutdown, but another parity check started soon after the reboot.
Oct 1 22:50:58 baconator kernel: mdcmd (45): check
Oct 1 22:50:58 baconator kernel: md: recovery thread: check P ...
Did you do start that one? Was it also a correcting check? Or was it actually rebuilding parity? Still ongoing 9+ hours later
Oct 2 08:00:01 baconator root: Parity Check / rebuild in progress. Not running mover
Then lots of read errors on disk3, finally the write errors that disabled it. Possibly Unraid tried to write the calculated data back to the disk it couldn't read. And finally it stopped itself.
Oct 2 12:48:51 baconator kernel: md: disk3 read error, sector=10938752032
Oct 2 12:48:51 baconator kernel: md: disk3 write error, sector=15780546472
...
Oct 2 12:49:27 baconator kernel: md: recovery thread: exit status: -4
My inclination at this point is to not believe in parity, but instead do New Config, and rebuild parity instead of rebuilding disk3. After you have fixed your hardware, of course.
The main problem with that idea would be if disk3 was corrupted somewhere along the way but that should become apparent immediately when starting the array. Or you could New Config without parity initially just to check if disk3 mounts, but not sure there would be much point since parity would definitely be out-of-sync that way. If there was disk3 corruption that could be dealt with separately.
The SAFEST approach would be to rebuild disk3 to a new disk and save the original disk3 and decide later which version of disk3 looked the best. Of course, that would require another disk.
See if there is anything else you would like to add, and maybe wait to see if someone else has a different opinion or approach.