itimpi

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.

It is probably your very restrictive value for the shares Split Level setting that is causing your problem.   The Split Level setting will over-ride the Allocation method and Minimum Free Space setting in the event of contention between them as to what drive  to use (as documented 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).

You can if you want but there is no need to as the plugin still functions as it has on earlier releases.   The plugin provides more extensive capabilities than the built-in support but if you do not use any of these extra options then less reason to keep it.

@tTownTom  have you checked the permissions at the Linux level on the share and the files in it?  There have been a number of reports of permissions being wrong for network access and it only showing up after upgrading.
 
it might be worth posting your system’s diagnostics zip file to your next post in this thread to see if anyone can spot something there.

You might want to check what is in the system share on disk5?   If it is the docker.img or libvirt.img files then having the docker/VM services enabled will keep the disk spinning.

Unraid does not mind if you leave gaps in the assigned drives.  Many people find it triggers their OCD, but as long as it does not worry you can leave the gaps in the assignments.   When you start the array only the disk slots that have drives assigned will be shown.
 
Just in case it is relevant it is worth pointing out that Unraid does not care how/where the drive is connected as drives are identified by their serial number.  There is therefore no reason that the assignments to disk slots have to match the physical layout if that is not convenient.

That setting means it will not be visible or accessible on the network.    You need it set to Yes to see it on the network.

How much RAM do you have on the server?    You need a minimum of 4GB to successfully update via the GUI.   If you have less than that you will need to use the Manual upgrade process as described 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.

That will be because when you make that sort of change Unraid restarts Samba so any transfer in progress at the time gets aborted.     There has been discussion about changing this to just tell Samba to reread its configuration file but as far as I know that has not happened and I do not know of any immediate plans to make such a change.

You can rebuild each disk in turn if that is more convenient.   After the first rebuild succeeds then the array will be back in a protected state.

If when you got the original failure once you had described your problem and posted diagnostics so we could confirm there was nothing else obviously wrong we could have given you the steps to get Unraid to accept the original parity disk so that you could then probably get an error-free rebuild of disk3.
 
 In principle it is a good idea to ask for help before letting Unraid do anything you are not sure about.

This suggests a hardware issue.   I would carefully check your cooling (especially the CPU fan) in case it is thermally related.   Another possibility since you say you recently added new drives would be that the PSU is overloading trying to handle the additional hardware.
 
As was mentioned the syslog server should allow a software trigger to be eliminated.
 
Regarding unclean shutdowns you may find this section of the online documentation to be helpful.

That shows the extended test failed, so the disk should be replaced.

In theory one could do clever things by going under the hood and manipulating the underlying Linux file/folder level permissions, but in practice for most purposes file security and share level security end up being the same thing when using Unraid.

What are the sizes of your data drive and the parity drive?  Once the parity build has gone past the size of a data drive then that data drive can be spun down as it is no longer needed.