itimpi

It is covered here in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page.

This is expected behaviour!   Once a drive has been chosen Unraid does not switch to a different drive if it turns out there is not enough space for it.
 
There is a Minimum Free Space setting that can be used to set criteria for when the next file should go to a different drive.  It is normally recommended that this should be set to a larger value than the biggest file you expect to copy just to avoid such failures.

If you click on the icon for any container on the Docker tab you get a drop-down menu for actions to perform on that specific container (one of which is Start).

The moment you format a drive to create an empty file system you are going to get some space used to hold the file system structures.   Around 1% of the drive size is not atypical and I suspect this is what you are seeing.

Unraid does not care where drives are physically connected.  It simply recognises drives using their serial numbers.

Pre-clearing is a personal decision, and is a good stress test of a drive before introducing it into an existing array.    However in this case the first thing you are going to do is build parity which is going to read every sector off every array drive, and write every sector on the parity drive.   If a disk is having problems I would expect you to get errors while trying to build parity so the pre-clear feels a bit superfluous.

You might find this section of the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page to be off help in understanding Unraid write modes.

You copy the /config folder from the ‘trial’ flash drive back to the old one (omitting the trial .key file) to transfer settings to it and then boot the new server off the old (licensed) flash drive.

Prefer means you WANT the files to end up on the cache.    You need to use Yes if you want them to be moved to the array.    Changing to No stops new files being written to cache but any existing files are left there.
 
Doing this will not affect existing files.   If the files really are deleted then you must have done something else to cause this.
 
At this point I would suggest rebooting in case you cannot see the shares because something has crashed and if so rebooting would bring them back.
 
if there is still a problem you are likely to get better informed feedback if you attach your system’s diagnostics zip file to your next post in this thread.

Assuming you mean on the flash drive then as long as the .key file is for that specific flash drive it is the correct thing to do.
 
Not quite sure why you should be seeing this.  You are likely to get better informed feedback if you attach your system’s diagnostics zip file to your next post in this thread so we have a chance to see what is going on.

You seem to have the NerdPack plugin installed which is incompatible with 6.11.x series? 
 
Have you tried booting in Safe Mode (which disables loading plugins) to see if you get the same symptoms?   If Safe Mode works then you know a plugin is causing your issue.

Nothing obviously wrong in the SMART report.    You might want to run an extended SMART test on the drive to make sure.

This is true as far as writing to the array is concerned.
 
if you have SSDs then these are normally used in a ‘pool’ external to the main array.   Pools can be made redundant by having multiple drives in the pool, but you are still constrained by the slowest component.   There is little point in having a SSD as an array drive with a HDD as parity, particularly with the way Unraid updates parity as described here in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page.
 
With the drives you mention you could have a single drive array using the HDD.    Then you could have a single drive pool using the SSD to host VMs.docker containers and run them with the full performance offered by the SSD.   Since this configuration does not provide redundancy then you could could do periodic backs (frequency chosen by you) to make backups of the VM/docker files from the SSD to the HDD drive in the array.   Plugins are available that can automate this process.

You get limited interaction from Limetech in the forums - they are mostly about users providing support.
 
I suspect you are not going to get any information about this until Limetech are at least ready to beta test something.  My guess is that any early implementation is likely to simply provide ZFS as an alternative format for pools, but even that is speculation.

Sounds as if you are seeing a manifestation of the behaviour described here in the online documentation accessible via the 'Manual' link at the bottom of the GUI. 
 
If you are on Unraid 6.10.x or later you could install the Dynamix File Manager plugin and use that to move files instead of Krusader.  This plugin is aware of the issue and will use a copy/delete strategy to avoid it.

That disk looks very sick and definitely needs replacing ASAP

Docker containers have the software they use installed internally to the container.   If there is a problem with the PHP version then it is a container issue and nothing to do with Unraid.

With the fact that the NerdPack plugin is now deprecated then the warning from FCP probably needs updating so that it only issues the warning if the packages are not from Nerd Tools plugin.

Have you made sure you exported the share to make it visible on the network?   

If you are having problems with the USB Creator tool then you can try using the older Manual install method documented in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page.

You can use the Parity Swap procedure that is designed for exactly this Use Case.

The delays are probably occurring when flushing RAM buffers to physical disks.
 
you can never achieve anywhere near the native write speed of a drive as under Unraid a write operation always involves multiple I/O operations.    This is described here in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page.

Have you tried  the process described here for handling unmiountable disks in the online documentation accessible via the ‘Manual’ link at the bottom of the GUI or the DOCS link at the top of each forum page?   

There is no connection to your Unraid server unless you install the MyServers plugin.   The only slight exception is if you are running a Trial version of Unraid when a temporary outward connection is made to check the Trial licence when the array is started.  The Unraid server can only be accessed remotely if you have installed the MyServers plugin AND enabled that feature.   

Are you trying to boot in legacy or UEFI mode?  For UEFI mode the Unraid flash drive needs the trailing ~ character removing from the EFI folder name.