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According to the SMART reports on disks 3 and 4
No self-tests have been logged.  
You have a lot of disks and I suspect power issues. In addition to those adapters, are splitters also involved? How many disks per PSU cable?

I assume because you haven't yet formatted it, so it has no filesystem to mount.
 
after formatting it in the array, right?
 
Or am I wrong with all this and you have already put files on the disk?

Remove those lines. They aren't working and aren't needed.

You can go directly to the correct support thread for any of your containers by clicking on its icon and selecting Support.

Your cache-prefer appdata share has files on the array. You want those moved to cache when there is space, or deleted if duplicates.
 
Mover won't move duplicates, and nothing can move open files.
Disable Docker in Settings so those files won't be open. Set appdata to cache-only temporarily so mover will ignore it then run mover to move your m---a share to the array so cache will have space. Set appdata back to cache-prefer then run mover. Possibly you will have duplicates so maybe they won't all move. I see you are on 6.10rc2. If you were on rc3 or rc4 you could use the Dynamix File Manager plugin to help figure out any duplicates and delete whichever is appropriate. After you get everything where it belongs enable Docker in Settings.  

Wonder why you wanted to install unMenu? Are you following some really old guides? Probably best to check the dates of whatever you are reading. Most things more than a few years old are probably not relevant.
 
Your appdata, domains, system shares are on the array. You want them on fast pool (cache) and set to stay there so your dockers/VMs will perform better and not keep array disks spunup since these files are always open. You will have to disable Docker and VM Manager in Settings, set these shares to cache-prefer, and run mover.

You have a user share with '$' as its first character. I suspect that is the reason for the problem with the User Shares page.

Corrupt docker.img. Delete and recreate
https://wiki.unraid.net/Manual/Docker_Management#Re-Create_the_Docker_image_file
 
Reinstall containers as Previous Apps
https://wiki.unraid.net/Manual/Docker_Management#Re-Installing_Docker_Applications

Delete or rename network.cfg so it will use defaults then go from there.
 
Consider reserving IP by MAC address in router instead of setting static IP that way you can manage IPs for everything in the one place that matters.

You don't have a plex media server plugin, must be left over from before the days of docker on Unraid. Just delete the file.

You can stop the current parity build, then go to New Config and create a new array without those disks and rebuild parity.

This is a path in the user share named "domain", could be on any disk depending on settings for user share domain.
 
If you want to specify a disk path, that would be /mnt/disk#
New Config will let you assign disks however you want and when you start the array those assignments will be recorded. Parity will be built on any disk assigned to any parity slot.
 
Don't assign it to the array.

You should have disabled Docker in Settings. Not only would that have prevented dockers from autostarting, but it also would have kept the docker service from starting and recreating the folders specified in your docker.cfg. This is probably how you wound up with a path to cache even though you didn't have cache yet. And that path would have been created in rootfs (RAM) since there was no mounted storage at /mnt/cache.
 
And then mover would have moved all that data to that path in rootfs, possibly filling it, and rootfs is where the OS lives. Maybe your system won't even work now to get those diagnostics I asked for.

OK. Let's forget about getting a clear answer to how you got here. Since you have single parity, it may be possible to recover the single broken drive, but all the other drives, and only those drives, that parity is currently synced with must be present and assigned to the array.
 
Since this is single parity, the order of the disks aren't important, so the below instructions should work regardless of how you assign the disks, provided you assign parity to the parity slot and all the other disks except for the broken one are present and assigned to the array.
Install a new disk. Go to Tools - New Config, Retain All, Apply. Make sure parity is assigned to the parity slot, all other disks assigned to a data slot, including the new disk. Check BOTH Parity Valid box AND Maintenance mode box, and start the array. At this point, all disks will be accepted into the array just as they are. Stop the array. Unassign the new disk. Start the array in normal mode (not Maintenance) with the new disk unassigned. At this point, the missing new disk will be emulated from the parity calculation by reading all other disks. Then post new diagnostics so we can see if all the disks are mountable, including the emulated missing disk.
 

Put flash in your PC. Delete (or rename) config/network.cfg so defaults will be used.

/mnt/user includes files on cache and array. User Shares are simply the top level folders on array disks and pools. Files on the lowest numbered disk are shown in the user share if there are multiples. Cache is considered lower than the array. Not really a question of the age of the files but in this case newer seems a reasonable choice to keep.

Correct
 
 

Since it doesn't know what the filesystem is supposed to be you will have to do it from the command line
 
https://wiki.unraid.net/Manual/Storage_Management#Running_the_Test_using_the_command_line

Remove the mover tuning plugin

Correct. Parity has no filesystem so it is not a mountable disk.
 
Take note of the serial numbers, not sd. Unraid keeps track of the assignments by serial number. The sd may change, especially if you add, remove, or change disks in the future.

Nothing obvious.
 
(Probably) unrelated, but your appdata and system shares have files on the array. Why do you have 50G docker.img? Have you had problems filling it?
 
You could try removing that container, then Apps - Previous Apps will let you reinstall it exactly as before.

Probably filling cache as you did broke user shares. You may have to reboot to fix that.

According to syslog, emulated disk2 mounted. SMART for disk2 looks fine.
 
Rebuild is the correct thing to do now.

You need to use the same flash drive but configure it for legacy. You can do that from the webUI, Main - Boot Device - Flash - Syslinux Configuration, uncheck UEFI.
 
Or just rename EFI folder on flash to EFI-