itimpi

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.

Yes.
 
The size is the amount of parity to be checked.   With multiple parity drives they happen in parallel so the size is that of the largest parity drive (if they are different).

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.

Yes, and has been present for the several releases now.

The fact that in the special case of 1 data drive and 1 parity drive they are mirrors of each other is an accidental by-product of the algorithm used to calculate the contents of the parity drive.   The moment you add any additional drives to the array this will no longer remain true.
 
What I do not see is why you need them to be mirrors?   With 1 parity drive and 2 data drives you still have redundancy against any single drive failure.

All drives attached to the machine (except the boot flash drive) count at the point you start the array regardless of whether Unraid is using them.  Your example shows 7 drives attached.

It is probably best (and safer) to leave it set to /mnt/user and use the Exclusive Share option that is now part of the 6.12.x releases.
 
That gets the same performance advantages as would be given by /mnt/cache and is less likely to be something you might overlook in the future when making changes.
 

This is NOT anything to do with the pool you have called ‘Cache’.   It is that somehow you have also ended up with a User Share called ‘cache’.   If you look you probably have a folder called ‘cache’ on one of your drives.  You definitely want to fix this and not ignore it.
 

Yes.    You need to carefully define the start time so it can only occur once during the lifetime of a check.
 
if you use the parity check plugin then you also get control over other types of long duration array operations.   It will still, however, expect you to get the start conditions right for scheduled parity checks.

If using SATA->SATA power splitters then you do not want to split a SATA connection more than 2 way as the host end is limited by the current carrying capacity of its SATA connector.   If using Molex->SATA then you can get away with a 4-way split.

It is a standard Linux file system so will be readable on any system that can handle a BTRFS file system.
 
The container binaries and working data are stored wherever you tell Unraid to store them (the 'system' and 'appdata' shares being the default locations.   It is normally recommended that these are sore on a pool for performance reasons.   The USB drive also stores templates for docker containers you install via the Unraid Apps tab in case you need to edit their settings or reinstall the containers with their previous settings intact.
 
 
Definitely.   The USB stick stores all you Unraid specific settings in its 'config' folder and this would be required if you ever need to transfer to a new USB stick.  You want a backup made any time you make a significant configuration change. There are currently 3 standard ways available:
By clicking on the Boot device on the Main tab and selecting the option to make a backup (which is then downloaded as a ZIP file). By using Unraid Connect to have automated backups made in the cloud on the Limetech servers. By using the "Backup/Restore Appdata" plugin which has an option to make a backup to a location of your choice when it runs to backup docker containers working data.

As mentioned Dynamix File Manager would be the recommended tools for this.

Not sure.    In principle you want the fastest drive as parity.   Suspect this would mean one of the SAS drives but you would need to check their specs to be sure.