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That is a clean looking check with no errors reported so I would expect all data to reappear after running the repair.

This would have also formatted the emulated drive thus wiping its data.    You would have needed to run the procedure for fixing disk1 to make it mountable before attempting any format.

You need to run without -n and adding -L

No, it is always based on the number of folders in the files path.

You have the Minimum Free Space setting for the 'domains' share at 195GB and none of your drives have that much free space.

Rebuilding a drive will not clear an unmountable status.    The procedure for this 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. 

You can do a manual upgrade to the current 6.12.10 release by following the procedure documented here in the online documentation accessible via the Manual link at the bottom of the Unraid GUI.  In addition every forum page has a DOCS link at the top and a Documentation link at the bottom.   The Unraid OS->Manual section covers most aspects of the current Unraid release.
 
Having done that the purchase option should now work.    If you still encounter problems you will need to contact support as they handle all licencing issues.

I would certainly recommend removing stopping the array as with disk4 playing up it will take forever 😞
 
It is your choice as to whether you want to try and resolve why disk4 is having issues or initially just get disk3 up and running?  It is possible that disk4 just has a cabling issue (power or SATA) and the drive is physically OK.

You should post your system's diagnostics zip file in your next post in this thread to get more informed feedback.  It is always a good idea to post this if your question might involve us seeing how you have things set up or to look at recent logs.

I would say that it is definitely your flash drive as the messages mention [sda] which is standard for the flash drive.
 
I do not think it will help in this case, but the key point of Safe Mode is that it stops plugins from being loaded (plugins can change system components) and that applies regardless of whether you are in GUI mode or not. 

Do you know which drives(s) are parity drives?   You will only lose data if you accidentally assign a data drive as a parity drive.   If you are not sure then follow the procedure documented here in the online documentation accessible via the Manual link at the bottom of the Unraid GUI.  In addition every forum page has a DOCS link at the top and a Documentation link at the bottom. 
 
Having said that you say the backups are on an unassigned drive?    Even with nothing assigned to the array I think you can still download the Unassigned Devices plugin and then mount such a drive to get at your backups.
 

No.   The disks will be Cleared so that parity is unaffected.

A gigabit LAN silently downgrading to 100Mbps normally means that there is at least one twisted pair not making connection in the cable between the router and the server.   This can be either a broken wire within the cable or a pin not making contact at either end.  I would suggest trying different cables and/or ports.

You can always use the Custom TAB to add anything as long as you have a URL for it.   I personally use it to add a DOCS tab there.

Did you copy across the contents of the 'config' folder from the old flash?    That should have gotten the plugins and docker templates back just as they were.

It is definitely possible to move the data.   
 
However are you sure the drive has failed?   More often then not a drive is disabled (because a write failed) due to an external factor such as cabling or power.  You should post your system's diagnostics zip file in your next post in this thread to get more informed feedback.   You can also try running an extended SMART test on the drive as if it passes that the drive is probably OK.

You can use the New Config tool to reset the array 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.   I recommend that you start by using the options to keep all current assignments to reduce the chance of error and then return to the Main tab to remove the drive before starting the array.   Since you have removed a drive parity will need to be built sgain.

You need to set either the “mirror to flash” option or set the “remote syslog server” to be your Unraid server to get anything recorded as mentioned in the syslog server link.   The settings you have just set it up in ‘listen’ mode

In theory reallocated sectors are not a problem as long as the number remains stable, but they are never a good sign, and if they continue to increase the drive will need replacing.   Definitely want to keep an eye on it.    Might want to consider running the extended SMART test on the drive as that is a good way to test it’s health.
 
if you click on the orange icon for the drive on the Dashboard and select the Acknowledge option Unraid will only notify you again if it increases.

With these types of problems it can be difficult to pin down the culprit. 😖   You might want to consider backing up the cache drive; reformatting it to get a clean file system; and then restoring its contents.   You can also get docker containers starting to play up after they have had an upgrade.
 
I mentioned RAM again because it is definitely possible for motherboard components to gradually degrade over time so that a system that has been stable for ages suddenly starts getting unpredictable errors.

The whole idea of Unraid (and RAID in general) is to minimise the chance of loosing data if a drive fails.  
 
In terms of backups many people only back up some of their data that they would find to be really painful if it got lost or is irreplaceable.   That may not be that large an amount.   You may not bother to backup large media files (particularly if you have a way you could replace them if really needed).

The easiest thing would be to download the current 6.12.10 zip file and then copy over all the bz* type files from the zip.    As a side-benefit you will be upgraded to the latest release.  At the moment this seems the most stable release.

If you read that link carefully you will see that despite its name it is NOT for replacing an existing parity drive with a larger one.   For that you simply follow the procedure for Upgrading parity disks.

You can install the plugin with the check already running.   Not much point in stopping the current check as you would just need to start again from the beginning.   The plugin is not responsible for initiating the operation (which is Unraid’s responsibility), it just adds better management of what happens after it is started.
 
Yes, but it will run much slower than when the check is not running (and also slow down the Check while they are both running) due to contention on the drives.  You should therefore avoid doing large amounts of writes to the data drives while a check/sync is running as doing them separately would end up being faster, but small amounts would be fine.
 
When you add a drive, parity is not regenerated.   Instead the drive is Cleared (if not pre-cleared) to avoid it affecting parity.   You can find a discussion of Clear v Preclear 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. 
 
 
 
 
 

The correct process for adding new disks is 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.    As you will see the last step was to format the drive which you said you did.   The fact that the subsequent rebuild resulted in an empty file system suggests that the format succeeded so at that point you should have been good to go without the rebuild.
 

there was definitely no need to Preclear the drive which is why it is not built into the core Unraid system.    The link above discusses Clear v PreClear.   In this case of an old parity drive being reused as a new data drive a preclear just wastes time.