Disk disappeared from vdev

I am new to TrueNAS and maybe did something wrong, but cannot figure out what:ň

I have set up a new installation, created 2 vdevs with 3 disks each in a RAIDZ1 config. Started adding some datasets, and suddenly i got some unspecific errors. Looking at the storage tab, I have a degraded vdev due to unavialbale drive:

At the same time I have a unassigned disk somehow

Not sure how this happened, but how do I add that disk back to the vdev?

See this doc, I think:
Replacing Disks | TrueNAS Documentation Hub

Are these disks behind a raid controller?
Noticed it has 7 errors- is it an old disk?
Have you done a SMART test on it?

the disks are used enterprise SAS drives, their smart seems OK. Also they are behind a HP P840 Smart Array controller in HBA mode.
I know about the replacement process, but adding the same disk back seems kinda pointless. If the disk is bad it should be shown as such and not as missing. If the disk got disconnected and reconnected for some reason, shouldn’t the vdev recover from it?
Readding a formatted disk is not possible, so I would need to erase it. It’s not an issue now, because the pool is empty, but the thought that this would happen everytime I pull out a disk from the running system does not make me comfortable.

Some supposedly Hardware RAID controller’s HBA mode are not a complete pass through. I can not say if the P840 Mart Array controller in HBA mode will work or not.

However, Hardware RAID controllers are a known issue with some of ZFS’ data integrity methods, (in order writes, write cache flushes when commanded, passing SMART through). (That last, SMART pass through is more TrueNAS than ZFS… for detecting disk pre-fail.)

Here is the reference;

It has not been copied to this forum, but the information is still quite valid.