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When it happens again click the Compute All button as seen in your first screenshot to see how much of each disk each user share is using.

Remove My Servers plugin

This is the wrong way to think about parity. Any bad drive in your array puts data at risk. Parity by itself cannot rebuild anything. Parity PLUS ALL other disks is required to rebuild a failed disk.

Also you might consider doing memtest.

USB connections are often unreliable and if a disk disconnects it will have to be rebuilt. This may be a frequent occurrence. Also, parity operations need to happen in parallel, so if disks are sharing a single connection performance will be severely impacted.

Sounds like a hardware problem. How are your power and cooling?

https://wiki.unraid.net/Manual/Troubleshooting#Persistent_Logs_.28Syslog_server.29

You can change the Mover schedule, but Mover is intended for idle time. Just write directly to the array and it will already be protected by parity.

3TB is the correct answer for your configuration: https://carfax.org.uk/btrfs-usage/ You would need to use Single mode to get full capacity from different sized disks:

It is impossible to move from cache to array as fast as you can write to cache, so if you intend to write more than cache can hold, don't cache.

The Diagnostics are a single ZIP file. Please don't post the contents of the ZIP, just the complete ZIP

Run an extended SMART test on that disk.

Tools - Diagnostics

USB NOT recommended for array or pool disks due to unreliable connections.

Have no idea what you mean here. The user shares are just another view of the disks, there are no pointers involved.