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Also

No, it is the OS in RAM. Edit config/docker.cfg on the flash drive to disable dockers and reboot.

On second thought, you may not have enough RAM to even run dockers. After rebooting in SAFE mode go to Settings - Docker and disable dockers.

You have filled rootfs, probably a misconfigured docker. Reboot in SAFE mode. What dockers do you usually run?

But the docker templates are on flash, so you will have to setup the mappings, etc. again. If you had the templates then you could use the Previous Apps feature on the Apps page and it would use those templates to setup your dockers exactly as they were. Having the docker image without the templates doesn't really help. The dockers executable code is in the image and they will run but you can't manage them from the Unraid webUI without the templates. The appdata is still good though since that will have everything the applications were doing. The dockers will have to be setup and installed again so they use that appdata, etc.

That is what I always recommend. Some even go so far as to not assign parity until after the initial data load, since parity updates make writes to the array somewhat slower.

If you look at your User Shares, you should find that appdata itself is a user share.

And if you are still transferring to a cached share while mover is trying to move then things can really go off the rails. Mover is intended for idle time. There is no way to move from cache to the slower array as fast as you can write to cache, and trying to move more frequently won't help anything if you are still writing. It will just be more competition for the same disks. Better to not cache at all when transferring large quantities of data unless cache has the capacity to take all of that data.

You might want to reconsider your opinion. If cache has less than minimum when it chooses a disk to write for a cached share, then it will choose to write to the array instead. If cache has more than minimum when it chooses a disk to write for a cached share, then it will choose cache. The choosing is done before the writing of the file begins. If you set cache minimum "quite low", and cache has more free than that "quite low" minimum, then it won't overflow, it will choose cache. And if the file you are trying to cache is larger than remaining free on cache the write will fail. You should set Minimum Free for cache to larger than the largest file you expect to cache.

Settings - VM Manager

Reboot. Make sure you are using a USB2 port.

That is not the diagnostics. Diagnostics includes log but also other things. Tools - Diagnostics.

Can you get any other machine to boot from the flash drive you have prepared?

That doesn't seem like an appropriate platform for a NAS. USB drives are not recommended in the array, and neither are SSDs.

Tools - Update OS - Branch Next https://forums.unraid.net/bug-reports/prereleases/unraid-os-version-690-beta22-available-r955/

Rebuilding a disk will result in the same bits that were on the disk, including any filesystem problems. Parity doesn't (can't) know about files or filesystems, just bits.

If this was your question this is the first time you've put it in those words. Free support is from your fellow users on the forum. Everyone who has responded on this thread is a fellow user, unpaid volunteers just trying to help other users.

Do you have an attached keyboard and monitor?

Is this a Ryzen system? Have you seen this?

Unraid sets corrupt flash read-only to prevent further corruption.

What does the GUI say your IP address is? (upper right corner)

Is the new system Ryzen?