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If an array drive gets below Minimum Free for the User Share, Unraid will select a different array drive.
 
If cache (or other pool) gets below Minimum Free, new writes to the pool will overflow to Secondary storage if one is set for the User Share.
 
For example, if Minimum Free is set to 50GB, and a disk has 60GB free, then Unraid can choose the disk for the file. If the file is 55GB and the disk is chosen, the write will succeed, after which the disk will only have 5GB free, which is less than Minimum. So the disk won't be chosen again for that User Share.
 
If Minimum Free is 50GB, and a disk has 60GB free, then Unraid can choose the disk for the file. If the file is 70GB, the write will fail because the disk doesn't have enough space.
 
In the general case, Unraid has no way to know how large a file will become so it decides in advance where to put the file, and it won't change its mind and try to move it somewhere else if the disk fills up.

https://docs.unraid.net/unraid-os/release-notes/6.12.4/#fix-for-macvlan-call-traces

CRC errors are just communication problems logged by the disk firmware. They are almost always connection or cable problems. You can acknowledge any SMART warning on the Dashboard page by clicking on it and it will warn again if any increase. I usually just acknowledge the occasional CRC, maybe reseat the connection next time I'm in the case. If it continues to increase investigate further and replace cables or whatever. Power cables and splitters can also be a reason for problems. Connection problems are much more common than actual disk problems so be careful with connections.
 
You are correct, you don't need parity swap procedure.
 
The documentation talks a lot about replacing a data disk, but replacing a parity disk is exactly the same.
 
You can't change disk assignments with the array started.
 
With the array stopped, assign the new disk to the slot to be replaced. Start the array to begin rebuild. Simple as that.
 
Shutting down, installing new disk, possibly removing old disk, whatever you need to do to get to assigning that new disk then starting the array.
 

I would run a non-correcting check just to see if everything is working well. If it doesn't have a lot of sync errors or any other problems then a correcting check.
 
Why do you have 200G docker.img?

Have you actually formatted any of your disks in Unraid yet? Doesn't look like you have any filesystems to create anything on.

Correct. You must let New Config rebuild parity (don't check the parity valid box).

https://docs.unraid.net/unraid-os/manual/storage-management/#rebuilding-a-drive-onto-itself

You have one rebuilding and one missing disk, which is all 2 parity can do, so nothing is protected if another disk fails.
 
Doesn't matter if a disk is empty or not, an empty disk is not a clear disk. It still contains the metadata for the empty filesystem on the disk.
 
Even if disk6 happened to be a clear disk, Unraid won't remember whether or not all of its bits are zero so considers it to have bits that are part of current parity.
 
So you currently have no redundancy in your array.

A container you have already installed from the Add Container page stores its template on your flash drive in config/plugins/dockerMan/templates-user. They are XML you can examine.

Just put the registered .key file back in the config folder on flash.
 
If you don't have a copy of the .key file contact support.

Mover Tuning Plugin

The config folder from your flash has all of your configuration. I would edit disk.cfg to set the array to NOT autostart so you can confirm your drive assignments.

Problems communicating with multiple disks. Power would be a suspect. Are there power splitters involved?

Change Docker from macvlan to ipvlan.
 
Check filesystem on disk1.
 
Rush named pools 👍
 

USB not recommended for array or pools for many reasons.
 
Attach diagnostics to your NEXT post in this thread.

Nothing indicates the drives need to be replaced. You just have a corrupt filesystem on the pool.
 
If you're going to reformat cache to fix the corrupt filesystem, you need to get everything off it you want to keep.
 
In your diagnostics, logs/syslog.txt, starting with these entries:
Dec 16 21:17:00 Tower kernel: BTRFS critical (device sdb1): unable to find logical 9836280312331550720 length 4096 ### [PREVIOUS LINE REPEATED 47 TIMES] ### Dec 16 21:17:00 Tower kernel: BTRFS critical (device sdb1): unable to find logical 9836280312331550720 length 16384 Dec 16 21:17:00 Tower kernel: BTRFS critical (device sdb1): unable to find logical 9836280312331550720 length 4096 Dec 16 21:17:00 Tower kernel: BTRFS critical (device sdb1): unable to find logical 9836280312331550720 length 16384 Also, just after those entries (and also somewhat before) you have dumps related to macvlan. These have been known to cause crashes. The solution is to change Docker from macvlan to ipvlan. But we can deal with that later since you should
 

rootfs is where the OS files are (in RAM as mentioned). If you fill that up, the OS can't work with its own files, so all sorts of things can quit working.
 
You need to figure out what is writing to RAM.
 
Often a misconfigured host path for a container will be the cause.

Your appdata, domains, and system shares have some or all files on disk1.
 
Ideally, these shares have all files on cache or other fast pool with nothing from them on the array. If these are on the array, Docker/VM performance will be impacted by slower array, and array disks can't spin down since these files are always open.
 
Set these shares to Primary storage: cache; Secondary storage: array; Mover action: array --> cache.
 
Nothing can move open files so you have to disable Docker and VM Manager in Settings then run Mover.
 
You can see how much of each disk or pool is used by each user share by clicking Compute... for the share or Compute All button on the User Shares page.

Looks like your cache drive has disconnected. Possibly related, your system share has some files on disk2.

You can consolidate free space to one drive using some of the features of the unBALANCE plugin, or move things yourself using the Dynamix File Manager plugin.
 
It is possible to create multi-disk pools outside the parity array using btrfs:
https://docs.unraid.net/unraid-os/manual/storage-management/#why-use-a-pool
 
If you want to combine the capacity of multiple disks of different size, you would use the Single profile:
https://docs.unraid.net/unraid-os/manual/storage-management/#change-pool-raid-levels
 
Pools can also be ZFS but I have no experience with that.

That looks fine, says 65% used, are you still getting that 75% warning? As long as it's not growing it's OK. You might increase allocation to 30G if you want to keep the warning away. Then if you get it again we can take another look.

Are you sure you don't have the array started in Maintenance mode?

Note that none of the ways you worked on this problem earlier would fix unmountable.
 
Be sure to post the output of check filesystem so we can advise further.

Note that replacing the disk won't fix unmountable filesystem.
 
Since this is the last drive in the array, was it recently added? Did you let Unraid format the disk after you added it to the array? Do you think the drive had any data on it?