Squid

Reseat all the cabling to the drives at both ends.  

Try deleting /config/plugins/corefreq.plg from the flash drive and reboot
 
If that doesn't solve it, then try safe mode

the appdata share vs any other random share is simply a convention and nothing your particularly have to stick with  Change the default appdata location in settings - docker.  This will NOT however change anything already installed.
 
The docker image does not exist on the flash drive, so if you boot with another flash and the image location etc is the same, then your apps will all show up (but won't be editable etc without the applicable files on the flash drive.  This includes references to the icons (Admittedly, this is a scenario I never tested with the various changes to the way icons are handled). 
 
The docker image is very safe to delete (Settings - docker with the service disabled) and then recreate (via Apps - Previous Apps), assuming you did an actual backup of your original flash drive which had the templates stored on it.
 

Just an error from the controller basically stating "Huh! The drive was here and now it isn't"

By using Krusader, you are effectively bypassing the "system", resulting in this behaviour
 
The way that every OS in existence operates on a move is that first the file / folder is attempted to be renamed to the new location.  Only if it fails does the copy / delete process (physical move) take place.
 
Because Krusader is working from the single mount point (/mnt/), the rename will always succeed, resulting in files being location in apparent violation of the rules.
 
Options to avoid this is once 6.10-rc3 is released to install a File Manager plugin at the same time, or 2 do file operations via Windows and move between the shares, or map the shares to Krusader individually and move between those mounts

rename the file to be docker.img instead and try again.  I seem to recall an issue with that.

Which ever gets you back to 6.9.2 Alternatively you can always download the zip file from the webpage (downloads) and extract all of the bz* files from the zip and replace them on the flash drive and reboot.

Run the check filesystem against disk 5
 

https://unraid.net/contact

The error is quite simply that the particular file is corrupted.  Because of privacy, actual XML's are never going to be included in the diagnostics, so it can't be determined exactly why, but based upon the fact that you're actually running the OS, and the file names within the diagnostics appear to be OK, then hopefully that's your only issue.
 
The solution is to either edit the file to see what's wrong with it and fix it or simply delete it.  The interesting thing is that the file listed is ..../templates/zigbee2mqtt.xml  This means that you've added (in Docker - Template Repositories) another entry other than the default one to point to the author's repository.  With CA, this isn't necessary at all.  Your best bet is to remove that additional lines you've added in Template Repositories so that this issue won't re-occur on the next check for updates.  (The "corruption" could simply mean that whatever repository you've added has a syntax error in it, and isn't in fact "corrupt")

It did definitely drop.  If problems continue, transfer to another flash.  Sandisk FITs run hot due to their very small size

Some routers (including yours) try and be "helpful" by purposely attempting to hack into your devices on the network.  It can be disabled in the router's setting (don't know what it's called though)

Corruption on that file (docker.cfg) at the very least
LOCKER_ENABLED="yes" DOCKER_IMAGE_FILE= 'mnT.user/system/docker/docker.img" DOCKER_IMAGE_SIZE<b40" DOCOER_APP_CONFIG_PA]H="/mnt/user/appdata/" DOCKGR_QPP_UNRAA_PATH5"" DOCKER_CUSTOM_NETGORKS=" " DOCJER_TIMEOUT}"10# DOCKER_LOG_ROTAION="yes" DOCKER_LOÇ_SIZE="µ0m" ÄNCKER_LKG_FILES="1" DOCKER_AUTHOBING_MMDE=#no" DOCKER_USER_NETWORKS="remove"  
Restore the flash drive from a backup.  Or if none available, delete the file from the flash and reboot (it looks to be basically stock anyways)

/config/plugins/dynamix/dynamix.cfg on the flash drive

Here's one issue.  Not quite sure if it'll affect it or not
 
From your Global Share Settings, 
shareUserInclude="disk2" shareUserExclude="disk2" You're telling the system that user shares should only be put onto disk 2 and you're at the same time telling it that user shares should never go onto disk 2
 
So where are they supposed to go?  Fix Common Problems should have been telling you about that.  Only ever use include or exclude, never both as they are opposites of each other with it never being necessary to specify both.
 
Fix that, reboot and post a new set if problems continue
 
 

It would sound like the flash itself is pooched (unless there's a physical write-protect switch on it).  I would transfer the registration https://wiki.unraid.net/Manual/Changing_The_Flash_Device

You're being bombed with advertisements for IPv6 addresses from DHCP.  Don't know why, but wondering if static addresses being assigned won't help

Odds on you slightly wiggled the cabling to the other drive...  No one ever in the world has ever said sata is a good connector (or the phrase "as pretty as an airport")
 
Reseat the cabling and start over again.
 
 

Perhaps the mover tuning plugin has the ability to selectively move based upon file type.  Not quite sure.  The base OS makes no provisions for this.

The share named "D.......s"?
 
# Generated settings: shareComment="" shareInclude="disk2" shareExclude="" At some point you had a disk 2 in the system, set the share to only use that drive.
 
Go to the share settings and reset that setting to disk 1 (or ideally all)

Yes, a 20GB docker image is the default set up.
 
It's the actual applications (or in Windows terms it's what's stored within c:/ProgramFiles).  In terms of what an "image" actually is, think of an ISO that you mount on Windows that you can read and write to.
Yes.  (Although this is configurable by you in changing the /config path mapping -> hidden option on most apps).  In terms of Windows it's somewhat akin to C:/ProgramData
 
 
The beauty of how Unraid, docker, and the Apps tab handles things is that at any given random point in time it's 100% safe (in the vast majority of cases) to recreate the docker image file, as it only stores the executables for the apps (Plex) itself.  Everything else is stored in appdata (which is untouched by the process), so after recreating it (and reinstalling from Apps - Previous Apps) it's all back the exact same way as it was originally, with no configuration changes on your part at all.  On average I redo my docker image about once every 3 months.  Not because there's any problems with it, but rather to ensure that the Apps tab works properly in this fundamental feature of it.
It really is.  Stop the service, delete the image, re-enable the service, followed by Apps - Previous Apps.  Use the check boxes and then hit Install at the bottom.  (If you've experimented with multiple plex containers, there may be more than 1 listed, but only check off what you were actually using (you can delete the ones you weren't using).  
 
Total its a couple of clicks and the speed of your internet.  But, you may be correct and I understated the time.  Call it the amount of time to start the process and then grab a coffee and return.
 
My opinion is yes. 
The docker container is part of the docker image.  Terminology is always a difficult thing.  As an example, I try and force myself to refer to everything as an "App" because it makes more sense to everyone with a cell-phone, but the correct term is "container", and sometimes I will bounce back and forth between the two without noticing.
 
Yes.  You tell Plex that your media library is at /data  Similarly, you tell Plex in it's settings that your temporary transcode directory is at /transcode
 
The concept of mappings, containers etc throws everyone for a loop.  It is also paradoxically concepts that once you understand it it's hard to explain everything.  This is a good video however.
 
 

It *appears* that the docker image is completely empty.  Apps - Previous Apps, check off everything you want and hit install.
 
I see that you're currently restoring the appdata.  While it's not a problem to store the docker.img within appdata (usually it's stored within system), it is 100% excluded from the backup set because its just too easy to simply recreate it.
 
Can he hit the mediashare via \\192.168.50.217\Media ?

At the time of the OOM, nano was open and was consuming ~15Gig.  Quite possible you opened the wrong file and then did the correct one, but it was the cause of the OOM

It *appears* that the flash is actually failing.  Nothing but critical medium errors throughout the logs.  Doesn't appear to have ever dropped offline, so changing ports isn't going to do anything for you.
 
I've never been a fan of any super super small flash device like the Ultra Fit's.   They are USB3 (run hot), and a very small form factor (run hot).  Run hot + run hot = very hot
 
I'd replace it (but avoid SanDisk as there are now a ton of counterfeits making the rounds)