Squid

You want to look at the allocation method for new files on the share settings or specifying the included or excluded disks on the share.
 
But, you can move the file from disk 1 to disk 2 via dynamix file manager or even unbalance (both available in apps).   The OS won't move it for you between array disks automatically

Enable debugging in Settings - Community Applications
 
Go back to Apps.  After the error re-appears, click on Debugging at bottom left and upload the zip file here.
 
(But update the plugin via the Plugins Tab first)

Unraid starts fresh with every reboot, so you need to save the changes on the flash drive somewhere and then re-apply them via the go file (/config/go) or user scripts when the system starts up.

With the diagnostics being anonymized it's hard to exactly tell, but I'd get that the downloads are either going to D--a or  D-------s share which both have files on the array, which will significantly degrade your speeds...
 
a-----------p shareUseCache="no" # Share exists on disk7 appdata shareUseCache="prefer" # Share exists on cache b-----s shareUseCache="no" # Share exists on disk5, disk6 C--------------------------------pshareUseCache="no" # Share exists on disk6 d--a shareUseCache="yes" # Share exists on disk1, disk2, disk3, disk4, disk5, disk6, disk7 domains shareUseCache="prefer" # Share exists on cache D-------s shareUseCache="prefer" # Share exists on cache, disk7 g-------e shareUseCache="yes" # Share exists on disk1, disk6 H---------s shareUseCache="no" # Share exists on disk2, disk3 isos shareUseCache="prefer" # Share exists on cache J----------e shareUseCache="no" # Share exists on disk7 l-----t shareUseCache="no" # Share does not exist M---a shareUseCache="no" # Share does not exist m------s shareUseCache="no" # Share exists on disk5, disk7 M-------t shareUseCache="no" # Share exists on disk4 P--------------e shareUseCache="no" # Share exists on disk3, disk5, disk6 P----s shareUseCache="no" # Share exists on disk1, disk2, disk3, disk4, disk5 system shareUseCache="prefer" # Share exists on cache T-----------e shareUseCache="no" # Share exists on disk3, disk4, disk5, disk6 u----------p shareUseCache="no" # Share exists on cache v-------p shareUseCache="no" # Share exists on disk7 w---------k shareUseCache="no" # Share exists on disk6, disk7  

Delete /config/network.cfg on the flash drive, reboot and reset things back up (this will revert you back to DHCP if you were using a static IP)

The USB creator has always had some issues on certain systems with various drives.  There are some workarounds online.  However, the manual method 99.9% of the time works with every drive so long as its 32G or less.  https://docs.unraid.net/unraid-os/manual/changing-the-flash-device/#manual-method
 
Realistically, the manual method takes pretty much about 1 minute to accomplish and the only real advantage of the USB creator program is that you don't need to right click and select "Run As Administrator"
 
 

Drive won't complete the self-test due to errors being found.  Currently 23 reallocated sectors present on a drive that's less than a year old.  I'd RMA it

The docker image exists on the array (share Data).  There is a huge speed penalty for this.  You really should put it into the system share instead which will create it on the cache pool.  (Settings - Docker.  Stop the service.  Delete the image, then change it's location to /mnt/user/system.  Renable the service then Apps, Previous Apps and check off what you had installed - all your appdata will be fine and it will be like nothing happened except for a nice speed increase)

Works no problems for me.  Browser extension?  I'd really need you to post in the CA thread with CA's diagnostics (Settings, Community Applications, Enable Debugging) then go to Apps.  After ~5 minutes go back to Settings, Community Applications and download the debug log and then post that in CA's thread
 

Close all browser tabs / windows on all devices that may be open to the WebUI (especially Android devices) and then try again.

Transaction: Memory read error That says it all.  One of your sticks is bad.  Possibly your BIOS' System Event Viewer would have more info on which stick needs to be replaced

Both your appdata share and system share exist on both the cache drive and disk 1.   
 
Stop the docker service (settings, docker) and then run mover from Main and see if that makes a difference.
 
 

If I'd take a guess, a container is going nuts on logging.  (Crowdsec?)  The docker logs are getting continually logged every 10 seconds (a container restarting constantly?)

You haven't formatted the drives after adding them.  There's a checkbox on the Main tab to format all the unmountable drives.  Presumably you also realize that if any data is present it will be lost.

Look on the docker tab in advanced view and see what's using the most amount of memory and then limit it.  You have at least one container that's running java and IIRC it's a real pig on memory use.
 

Use the manual method of creating the flash drive.  Nothing really special about doing it.
 
https://wiki.unraid.net/USB_Flash_Drive_Preparation#Windows_Vista_and_Windows_7
 
Just make sure its 32G or less

Post your diagnostics  But, the first question is have you formatted the drives yet?  Are they all listed as being unmountable in Main?

Probably because until the process completes, there may still be references etc to the old drive.

UD mounts the share from ServerB within /mnt/remotes/someName.  To get plex to see it, you add another Path to the template (edit Plex), and give it a host path of /mnt/remotes/someName with a container path of something like /ServerBMedia
 
The within Plex, you edit the applicable library and add in /ServerBMedia to its list of folders.  Rescan the library and they should all be there.

You're not getting an IP address from your DHCP server (router).  Reboot the router / modem, or give the system a static IP in the appropriate range via Settings - Network Settings

The screenshots all show 4 cores which is what you have.  (Display aberration though where it thinks that 2 of them are HT cores, but that doesn't affect anything)

May 8 06:48:15 Backupserver mcelog: failed to prefill DIMM database from DMI data Harmless and safe to ignore.  A BIOS update though might silence it

You don't want to do that as you're not using a proxy...
 
Can you 
ping -c3 8.8.8.8  
Reboot the router, switch(s) etc

Short of a different flash or port, you can try this 

 
You will however have to have the flash working in order to install it.  (And then reboot after its installed)

You're using a folder for the docker "image".  But, you have it in the system folder which is set to use cache "Yes".  You want this to be use cache: ONLY, as the docker system probably can't make heads or tails when things get moved around (and you want it to stay on the cache drive anyways for performance)
 
Stop Docker service in Settings - Docker.  Delete the image folder and then re-enable the service.  Go to Apps, Previous Apps and check off whatever you had installed previously and hit the install button at the bottom.