JorgeB

Bad flash drive should not cause this, memtest is only definitive if it finds errors, if you have multiple sticks try with just one, if the same try with a different one, that will basically rule out bad RAM.

SMART test passed so the disk should be OK for now, keep monitoring, you can also replace/swap cables to rule that out if more errors occur.

You can get the key from the purchase email link, if you no longer have that, contact support.

Yes.
 
For the array we recommend xfs for the typical user, if you want checksums/snapshots for the array then I would recommend btrfs over zfs, since there's currently a known write performance issue with zfs (on the array only).
 
Zfs for pools is a good option, and for raid5 is the best option.

Try running another filesystem check, that still looks like a filesystem problem to me, if it doesn't help, I would backup and re-format that disk, to make sure the issue is resolved.

It's not logged as a disk problem and SMART looks fine, so most like a power/connection issue, replace both power and SATA cable and try again.

The issue is fixed with 6.12.8, the commit that caused the issue was reverted in the current kernel, and some users already confirmed there's no problem now.

Check filesystem on disk4, run it without -n, and if it asks for it, use -L.

It's possible, but since you have another NVMe device, swap slots between them and re-test, see where the issues follows.

First try to see if the problem is with the docker service or a container, I don't think you can disable autostart for any container with the service stopped, so possible better to just recreate the docker image and re-test.
 
https://docs.unraid.net/unraid-os/manual/docker-management/#re-create-the-docker-image-file
Also see below if you have any custom docker networks:
https://docs.unraid.net/unraid-os/manual/docker-management/#docker-custom-networks

Constant ATA errors with both disks, looks more like a power/connection issue, replace cables and post new diags.

Diags show errors with disk7, don't see any with parity, disk7 errors look more like a power connection problem, replace cables (also do it for parity) and run a parity check.

Problem is the USB enclosure, it's making all the devices have the same apparent serial number, note the end numbers are all the same, that should be the serial number:
Apr 3 20:35:04 unraid emhttpd: ST2000DM_001-1CH164_152D00539000 Apr 3 20:35:04 unraid emhttpd: WDC_WD40_EFAX-68JH4N1_152D00539000 Apr 3 20:35:04 unraid emhttpd: WDC_WD40_EFPX-68C6CN0_152D00539000 Apr 3 20:35:04 unraid emhttpd: WDC_WD50_00BPVT-22HXZT3_152D00539000  
Also note that USB for array/pools is not recommended for other reasons.

This.

You will also be able to add subpools to existing pools, also note that subpools can already be used with v6.12 (and can be imported by 6.13 later), but they need to be create manually:
 
https://forums.unraid.net/topic/46802-faq-for-unraid-v6/?do=findComment&comment=1256329
 

Try using an exclusive share (or disk share), both will bypass FUSE, which is known to impact performance a lot with small files.

docker.folder.plg - 2021.08.20  (Up to date)  (Incompatible)  
Remover this plugin, I believe there's a replacement now, but don't remember the name.

Yes.

Run a scrub and when done look at the syslog for the list of corrupt file(s), delete/restore them from a backup, then run another scrub to confirm no more errors.

Apr 17 19:40:52 palazzo kernel: mpt2sas_cm0: SAS host is non-operational !!!!  
HBA problem, make sure it's well seated and sufficiently cooled, you can also try a different PCIe slot, then post new diags after a reboot with the array started.

Try this:
https://forums.unraid.net/topic/123369-ssh-errors-operation-not-permitted-when-attempting-to-ssh-into-another-unraid-server/?do=findComment&comment=1126266
 

Since a new flash drive with a stock install works, you can try redoing the flash drive, backup the current one first and then redo it and just restore the bare minimum, like the key, super.dat and the pools folder for the assignments, also copy the docker user templates folder, if all works you can then reconfigure the server or try restoring a few config files at a time from the backup to see if you can find the culprit.

You can still use macvlan if you need it, but in that case need to disable bridging, see the 6.12.4 release notes for more info.

poll cfg is still showing as a mirror, try this:
 
unassign all pool devices
start array
stop array
assign only the old device
start array
stop array
now assign the new device together with the old one
start array
 
If this doesn't work post the output of
zpool import  

Make sure the LSI is using the latest firmware, if that doesn't help make sure the disk is spinning up, in case it's being powered differently, if still nothing, it could be a compatibility issue.