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You posted a syslog. That is only a small part of the complete diagnostics. Sometimes the least important or informative part.

Enter diagnostics at the command line and see if it will write the diagnostics to the flash drive so you can post them.

The reason I asked is because many people have problems booting or maintaining a connection to the flash drive in a USB3 port. So what is the answer?

Are you booting from a USB2 port?

Can you boot in nonGUI mode and get to the command line?

Post a picture of this

Put flash in PC and checkdisk. While there make a backup. Try again. Make sure to use a USB2 port.

Looks like nobody else want to chime in. I guess you might as well proceed with the invalidslot. Be sure to follow the instructions in that post exactly, except for disk3 instead of 4 as noted. Be sure to ask if you have any problems or questions.

When you do New Config before issuing the invalidslot it will let you assign the disks as you want. You must always have at least one other copy of anything important and irreplaceable. Parity is no substitute for backups. Plenty of ways to lose files besides disk failure, including user error. You don't have to backup everything, but you do have to decide what you can't afford to lose.

Looking again, I think it is probably just a connection problem with parity. Lots of errors in the syslog on disk0, and parity isn't showing SMART in your 2nd diagnostics. I think it looks OK in the 1st diagnostics though.

Parity disk doesn't look so good either. I would put disk3 and parity replacement ahead of disk1. What you need is the invalidslot command. Instead of 4 in the command in this linked post you would put 3 to rebuild disk3: https://forums.unraid.net/topic/77771-multiple-errors/?do=findComment&comment=720061 Take a look at that but don't proceed for a while to see if someone else has anything to add. I always like to ping @johnnie.black at times like this. Do you have another copy of anything important and irreplaceable?

You must have at least one data disk in the array. This FAQ talks about how to set up the different btrfs raid configuration for the cache pool: https://forums.unraid.net/topic/46802-faq-for-unraid-v6/#comment-480421

https://forums.unraid.net/topic/83894-full-nvme-ssd-only-unraid-with-asus-hyper-m2

Post your docker run command as explained in the very first link in the Docker FAQ: https://forums.unraid.net/topic/57181-real-docker-faq/

The Unraid OS will require some RAM so not a perfect split to only these. Parity (not required but recommended) must be at least as large as any single data disk, so the 6TB would be parity. The SSD would be cache. Do you have any data on these? Unraid must format any disk it will use in the parity array or cache pool.

I don't use any of these dockers, but in general, you can get to the console (command line) inside any of your docker containers by simply clicking on its icon in the Unraid webUI and selecting >_Console.

In other words, it should be cd /dev/dri ls

Still no memtest?

No matter how many dockers you have you should not need more than 20G unless you have one or more of your apps misconfigured. Making it larger than 20G will not fix the problem, it will just make it take longer to fill. I recommend to delete docker image, recreate it at 20G, use Previous Apps on the Apps page to reinstall your dockers as they were, and then figure out what you have wrong about your apps. The reason for filling docker image is usually some application writing to storage that isn't mapped. Typical mistakes are not using the same upper/lower case as you have in the docker mappings, or not specifying a path that begins with /

The correct way to replace a disk is to simply assign the new drive to the slot of the drive your want to replace, and the replacement drive will be rebuilt from the parity calculation so it has the identical contents as the original, replaced drive.

Did it have a red X next to it?

Are you sure that drive was the source of the noise? Typically there will be other symptoms. Were there? Do you have Notifications setup to alert you immediately by email or other agent when Unraid detects a problem? You might try opening it in 7zip on your PC.

That guide is about removing a disk, not replacing a disk. Removing a disk then adding a disk is not the way to replace a disk. Also, that "guide" is nearly 6 years old, and for a very old version of Unraid that I assume you are not running. That very old version doesn't even present the New Config options to you the same way as the current version of Unraid. New Config (which is not what you wanted to do) will let you keep your disk assignments, without screenshots or possible mistakes in reassigning them. And, even if you did actually want to remove a drive, those instructions are needlessly complex in my opinion, probably mostly due to the instructions being very, very old. You can do everything you need to do to remove a disk without going to the command line. Why did you think you had bad disks instead of some more likely problem such as a bad connection? Problems communicating with the disks, such as bad connections or controller issues, are the cause of the vast majority of disabled disks.

Why do you have 200G for docker image? That is 10 times the recommended amount. Are you using a USB drive for parity? USB, though permitted, is not recommended for disks in the array or cache pool due to unreliable connections with some implementations.

Unassigned Devices plugin