A disc died and I tried to rebuild the data with help of a used spare disc that was around.
The "new" disc had already partitions and some old data on it (not related to the nas).
So I followed https://wiki.unraid.net/Replacing_a_Data_Drive and at the "Assign the new drive in the slot of the old drive" it complained that the disc is too small.
So I wiped the new disc with
sgdisk --zap-all /dev/sdX
Afterwards, if I remember correctly, it formatted the new disc. I was able to use it with the unassigned plugin.
So afterwards I was able to add it at the "Assign the new drive in the slot of the old drive" step.
Likewise it showed the popup that I "ignored" because I did not formatted any disc and was not aware that I format anything unmountable.
So I ended up with a empty disc with complete data loss, realised it a week later so my memory is a bit fuzzy.
I google a bit what could gone wrong and immediately found that this is a common Unraid problem (seriously wtf).
At the next disc fail I would likely do it similar again.
I did in the past at several different NAS vendors (Synology, Qnap, some HW Raid) and never had a problem because the way to do it was crystal clear and self explained.
So what is the correct way if it complains for some reason like disc to small due old data on it?
That is not covered by the docs and acting with common sense leads to data loss.
Tbh I did not expect that Unraid tries to delete my files so I gave it not too much attention because I expected it just works like at all other NAS Software I used in the past.