itimpi

Deginitely worth checking out the power connections.    Do you use splitters to provide the power?  They can sometimes be problematical.

You are likely to get better informed feedback if you attach your system’s diagnostics zip file to your next post in this thread. It is always a good idea when asking questions to supply your diagnostics so we can see details of your system, how you have things configured, and the current syslog.
 
The syslog in the diagnostics is the RAM copy and only shows what happened since the reboot.   It could be worth enabling the syslog server to get a log that survives a reboot so we can see what happened prior to the reboot. The mirror to flash option is the easiest to set up, but if you are worried about excessive wear on the flash drive you can put your server’s address into the Remote Server field.
 
Do you by any chance have the S3Sleep plugin installed?   That has been known to spuriously shutdown servers.  

You cannot change the file system type in this way as the rebuild process works at the raw sector level and is file system agnostic so it can only rebuild the previous file system.   Stopping the array and setting the file system back to XFS and then restarting the array should with any luck bring back your data.
 
Changing file systems on a drive always involves copying the data elsewhere; reformatting the drive to the new file system (wiping it contents); and then copying the data back.

Each container can have its own settings for security on the its appdata folder and its contents.
 
You should be able to access it OK if you use the Dynamix File Manager as that runs with root privileges.

Your problem is due to the fact the pool is formatted as xfs and that format only supports single-drive pools.    If it had been formatted as btrfs then you could have easily added the drive.
 
to do what you want you need to back up the current contents of the pool to the array; delete the existing pool; set up a new 2 drive pool using btrfs.   Format the pool to btrfs and then copy the contents back.    By default the pool will be set up as raid1 with the usable space that of the smallest drive, but you can click on the drive on the Main tab and change it to use the Single btrfs profile which will give you the full 1.5TB of usable space.
 
 

According to the diagnostics all of your drives have dropped offline as unable to get SMART data from any of them so not much point in trying to continue the check.  At the very least you are going to have to reboot..   not sure what could take all drives down.
 

I suspect the Parity Check Tuning plugin is correct as it tracks the check using a different mechanism to the built-in Unraid method, and Unraid has been known to at time incorrectly display the result of the previous check.  You might also know what the true elapsed time of the check was as the two reports give very different values.?
 
 
Unfortunately you cannot do this in practice.  The sectors that have errors are listed in the syslog but there is no way to know which drive generated them, or even if you knew that easily relate it to any specific file.   The only way to find out is to have checksums for your file or to be able to compare them to your backups.   If using XFS in the array then the File Integrity plugin can be used to generate these, or if using BTRFS or XFS they have check-summing built in at the file level.   The algorithm used for writing drives is such is that barring errors (which the drives should report back to Unraid) the parity drive is the one most likely to be in error.
 
If you have had any unclean shutdowns then a small number of errors are to be expected.  If not then doing some hardware checks such as extended SMART tests on your drives or a RAM check are a good idea.

Those are symptoms of Unraid being unable to read its configuration from the flash drive.   That suggests either the flash drive is starting to have problems or something is causing it to drop offline or some corruption in the configuration information saved to the flash drive.
 
At the very least I would suggest plugging it into a PC/Mac and running chkdsk on it. You also might want to look for the presence of FSCK type files on the flash drive indicating earlier corruption found and an attempt made to fix it.   You should also take the opportunity to make a backup onto your PC/Mac.
 

I would suggest rebooting and trying again.   If it still does not work then post your system’s diagnostics zip file taken after the failed attempt.

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 of use.

Is the new system booting using legacy mode or UEFI mode?   If the latter make sure that there is a EFI folder on the flash drive with no trailing - character (rename it iif necessary).

Normally if you have a share of a given name then all top level folders of that name on any pool or array drive are combined to give a single unified view under /mnt/user.   Doing an rsync between a user share (anything under /mnt/user) and a drive share (which is what /mnt/backup_hdd is) can easily lead to data loss unless you are very careful and know what you are doing.
 
Have you read the section of the online documentation that discusses User Shares and how they are implemented?

See some syslog error entries I have not seen before relating to /dev/sdc (disk3) which is I assume the one you are trying to format.   It appears there is a failed attempt to update the partition table..   I also notice that you are on a very old Unraid release (6.11.2) and cannot remember if there were errors in that release that might be relevant.
 
it might be worth making a backup of your Unraid flash drive and then updating to the current Unraid release (6.12.10) and seeing if the problem still occurs.  You can use the manual update method as described here to do the upgrade.   Make sure all plugins are up-to-date, and if you encounter any problems immediately after updating you can boot in Safe Mode to see if they are plugin related.   At the very least that will mean you are on the current Unraid release which is likely to make further troubleshooting easier.

What address does your workstation have for its IP address and gateway?    The Unraid server needs an address on the same subnet and should use the same gateway(s).
 
If you cannot figure things out then deleting the network.cfg file; rebooting the Unraid server; and then taking and posting new diagnostics so we can see what happens in that circumstances might help.

This behaviour is documented here in the Caution 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 need to use a copy/delete strategy for moving files to avoid this.    If you use the Dynamix File Manager plugin to move files it will do this automatically.

That share has a Minimum Free Space setting of around 781GB, and none of your drives has that much free space.   

Unraid only supports booting off the flash drive (and loading the OS into RAM.   Not sure why you have to ‘interrupt’ the boot sequence - the flash drive should be set as the default boot drive in the BIOS to avoid this.

Have you read 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. It covers transferring the licence to a new flash drive.

It does not fix the error, but if you click on the orange icon against the drive and select the Acknowledge menu item then Unraid will only notify you again if one of the values increases.
 
As mentioned I would not be worried about the CRC error as that is a connection issue external to the drive and not an issue unless you get a lot.  The others are more concerning.

What you need is the parity swap procedure which is designed for exactly this scenario.   It allows you to upgrade a parity drive to a larger one and then use the old parity drive to replace the failed data drive.

No - you DO get all Unraid features (including VMs and Docker) and keep your lifetime update status.   All releases of Unraid are functionally equal, and the web site should make this clearer.

Looking through the diagnostics it seems the parity1 drive has gone offline and as a result there is no SMART information for it.   You might want to check the power/cabling to the drive.

The User Scripts plugin can be used to initiate the scripts on a selected schedule for devices permanently available.   Alternatively the Unassigned Devices plugin allows you trigger a script when a particular drive is plugged in.
 
The content of the script will depend on exactly you want to copy and to where.

Those look fine from a scheduling perspective.   
 
We do normal, however, recommend that the automatic scheduled checks are set to be non-correcting so that if you have a drive playing up it does not end up corrupting parity before you realise there might be an issue.   Then it is only the manual checks that are set as correcting after you have fixed any hardware related issues that might have caused parity to be partly invalid in the first place.
 
  
 
You can get to the flash drive by clicking on the Boot device on the Main tab.     If you want it to be visible over the network (by default it is not) then you need to set the SMB settings to allow this (in which case if shows up as the 'flash' share.
 

The end of both logs finishes with errors along the lines of:
Apr 22 08:33:48 Tower kernel: ata14.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6 Apr 22 08:33:48 Tower kernel: ata14.00: irq_stat 0x40000001 Apr 22 08:33:48 Tower kernel: ata14.00: cmd a0/01:00:00:00:01/00:00:00:00:00/a0 tag 7 dma 16640 in Apr 22 08:33:48 Tower kernel: opcode=0x12 12 01 80 00 ff 00res 00/00:00:00:00:00/00:00:00:00:00/00 Emask 0x3 (HSM violation) Apr 22 08:33:48 Tower kernel: ata14: hard resetting link Normally we would think these were power/sata cabling related but the fact it has occurred at the same point twice suggest it may really be a drive problem if ata14 is the new drive.

You can use the procedure 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. The Unraid OS->Manual section in particular covers most features of the current Unraid release.