Kilrah

Its size is the one you set for it on the Docker settings page, default is 20GB. If you want to shrink it you have to delete and recreate it with the new size.
 
https://docs.unraid.net/unraid-os/manual/docker-management/#re-create-the-docker-image-file

When you replace one drive there will be 2 sources of redundancy, so if one is unreadable it'll use the other. If both were bad the whole operation would fail AFAIK.
There are checksums so it can know if the data is correct or not. Either it rebuilds correct data or it can't and the pool is gone.

You should not enable XMP anyway for stability even when you're using faster RAM, so no, speed not important.
As to amount, depends what you do and need.

There isn't one. The flexibility is what you lose with a zfs pool, it's one thing that can't be broken down or (easily) expanded.
 
Do you have extra SAS/SATA ports to shuck the externals and connect them internally?

Siehe vorherigen Post und Unraid-Version prüfen... 

That is for appointing someone to configure your server for you. Licensing issues have nothing to do with this and will be solved when you fill up the form with your problem without any payment obviously.

Wahrscheinlich bist du auf Unraid 6.12.9, gibts da ein Bug mit Remote Mounts. Auf 6.12.10 aktualisieren

The beta is closed, you only have access if you registered for it from the email several months ago with your unraid account and were accepted.

Yes, would not put that drive in use with 8 newly reallocated sectors.

Your appdata share is set to 200GB min free space, both of your drives are below that at 114 and 188 respectively so anything trying to write to appdata will encounter a "disk full" error.
 
I.e. it was never the Docker image.

Do they not work as in the UI shows them stopping again, or you just can't connect VNC? If the latter just clear browser cache/cookies.

Would try recreating the docker image at this point. 
 
https://docs.unraid.net/unraid-os/manual/docker-management/#re-create-the-docker-image-file

Try using something like glances or netdata with persistence to see what container caused that runaway increase in RAM usage.
 
EDIT: Seems it could be immich, try limiting the amount of RAM it's allowed to use

Nein, immer gleich oder grösser.

Have you set up core isolation in Settings-> CPU pinning? If so then those wouldn't be used for anything else than VMs.

rsync from a user script

This only displays nominal speed, not actual. 
Use corefreq plugin if you want to see current clocks.

/tmp is not a mount so it's in the rootfs, that's half the installed ram and as you saw things will go south if it gets full and there's no space for the OS to work with anymore.
/dev/shm is a separate tmpfs in RAM with size being half the installed capacity too, better to use that since if it fills up it won't clog the rootfs that still has the other half to work with. 
 

Yes, because it has to ensure all currently unused bits are 0 to be ready for when you're going to install a larger disk that will use them.

I would add only this disk to array, no parity or anything else and start the array. If it mounts all good, if it says unmountable then stop again and remove from the array, you will have to copy.  Oops, didn't notice EXT4
 
Assuming you'll want a parity disk if the sizes are OK I would take that drive intended for parity, add it to array, format, copy everything from the disk that has data to it, then add the old drive as parity.

https://unraid.net/policies
 

All your containers will show up in "Previous Apps" on the apps page where you can just select and install them in bulk.

So you're now copying parity data onto that 6TB drive?
Looks like it's an SMR drive so... you're just going to have to be very patient. 
 
You might be able to gain some time by pausing the operation, waiting an hour or so then resuming, and repeating everytime it falls down to negligible speeds.

Probably those low-end SSDs, seems sub-50MB/s on extended writes that have filled the pSLC cache isn't surprising. 
 
https://goughlui.com/2023/03/11/review-patriot-burst-elite-1-92tb-2-5-sata-iii-ssd-pbe192ts25ssdr/

It's not but it only shows in Advanced view, also it seems you're using a folder plugin so if you're already in advanced view that could interfere with it/change where it's shown.