Hi i’m new to the forum 
I have a question.
I have a raid Z2 with 4 hard drive of 1tb and I would swap 2 hard drive of 8tb replacing the 2nd and the 4th hard drive.
I would know if is possible to do that without losing any data and in case how move the data.
Thanks!
Theoretically you can just physically replace the disks and then replace them in GUI, this is not an actual recommendation though. I just want to point out that with RAIDZ2 two disks may fail without data loss. You still want the redundant during resilvering.
Add one or both to your system, depending on how many free ports you have and then just replace one drive and then the other. It takes some time to resilver in between. Afterwards remove the replaced disks from the system.
Identify which disks you want to replace beforehand from the storage / disks tab via the serial number.
If you are on core look up the same section in the core documentation.
You will not gain any space until all four drives are replaced with larger ones! This is one of the drawbacks of a RAIDZ system.
cool so basically do like for example today the disk 2 and resilvering it
And after tomorrow or in 2 days the disk 4 and resilvering right?
It matter which disk i replace?
I can replace the 2nd and the 3th for example?
Also once I begin to reswap day by day all the 4th resilvering I will have at the end of the swapping operation 16tb right?
So meaning the only way to don’t lose data If I have understood is swap everyday a different hard drive and wait to have resilvered
No, any two disks on RAIDZ2 may fail.
Yes, replace all four drives one by one and then you will end up with ca 16 TB usable space.
If you could connect the extra 4 disks at once, you could do all the replacements at once as long as you don’t offline/remove the disks being replaced.
BUT you should burn in the new disks first.
I use @spearfoot’s burnin script
Run it in tmux.