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Go to Tools - Diagnostics and attach the complete Diagnostics ZIP file to your NEXT post in this thread.

Even more important, don't try to cache initial data load. Cache won't have the capacity and will just get in the way. But, you want to make sure to install cache before enabling dockers / VMs so those will get setup properly on cache where they belong.

The reason I said NEXT is so you would make a new post. Otherwise I wouldn't have noticed this thread again. Looks like disk7 may be a problem. Run an extended SMART test on it.

You should always Go to Tools - Diagnostics and attach the complete Diagnostics ZIP file to your NEXT post in this thread.

Why have you given 40G to docker.img? I suspect you have been filling it since 20G is usually much more than enough. I run 17 dockers and they take less than half of 20G. Also, your appdata, domains, and system shares have files on the array, so your dockers / VMs will have performance affected by slower parity, and dockers / VMs will keep array disks spinning since these files are always open. Not quite sure what to make of these statements. You start out saying you "ensured that" and end by saying "no clue".

Go to Tools - Diagnostics and attach the complete Diagnostics ZIP file to your NEXT post in this thread.

Also notice from the photo 2 +12V rails. Wonder what else might be a reason to worry about this "older computer"?

Could take some additional forum help to get through this so you may need access for several hours

memtest and safe mode are both on the Unraid boot menu.

/backuproot, which you have specified as both the host path and the container path for that mapping, doesn't correspond to actual Unraid storage. And that other path you tried to specify within the container itself doesn't correspond to anything inside the container. Do you understand docker volume mappings?

And DON'T FORMAT anything!!!

If we can get disk6 to mount then maybe we can try the rebuild again.

Format writes an empty filesystem to the disk, and Unraid treats that write operation just as it does any other, by updating parity.

Post your docker run as explained at this very first link in the Docker FAQ:

Have you looked at the files in there? You may find the filenames have also disappeared.

Preclear if you want to test it, but you must assign the new disk to a new slot in the array THEN let Unraid format it. If you preclear, format, then assign, Unraid will just clear it again to maintain parity since a formatted disk isn't clear.

Any idea why disk6 is showing as a new disk in that 1st screenshot? In syslog it looks like it wasn't mounting, and it seems you were having connection problems with disk6 during that whole rebuild. And no SMART report from disk6 which suggests it is still having connection problems. You must always double check all connections whenever you are mucking about in the case changing disks or whatever. Do you still have the original disk2 with its data intact?

In order for cache-yes to overflow to the array, Unraid must decide that cache doesn't have enough space. Since it has no way to know how large a file will become, it decides to use another disk based on the setting for Cache minimum free in Global Share Settings. While cache has more than minimum, it will always choose cache for new files for cache-yes shares. Each user share has a similar setting for Minimum. When a disk has less than minimum, Unraid will choose another.

https://wiki.unraid.net/Check_Disk_Filesystems#Checking_and_fixing_drives_in_the_webGui

Whether rebuilding parity disk or data disk it's the same. Read all other disks to get the result of parity calculation then write that result to the rebuilding disk.

As mentioned, Parity Swap may be the way forward. Maybe even if the disk isn't really bad since that is a way to go just so you can begin to increase capacity. Doesn't really get you any more capacity immediately since it gives you 8TB parity and 4TB parity replaces 4TB disk4, but with 8TB parity you can later replace disks with disks up to 8TB. I see you already found the wiki on Parity Swap, but before recommending you go ahead with that, it might be useful to check other things to see if there is anything that could cause problems while doing that. Diagnostics might tell.

@winglam I merged your threads. Not sure if you posted both times in the FAQ thread or not. Possibly the first was already moved from there. You should only post in the FAQ thread when you are giving the ANSWER to a Frequently Asked Question. If instead you made one of these posts in a different thread than the FAQ thread, then don't post the same question in multiple threads.

What exactly are you trying to do? Whatever it is you probably shouldn't be trying to do it at the command line.