Added the 4th disks but it is not RAID1

cyse wrote:
Thank you everybody for your clarifications, I don't really understand how xraid2 works... I have 4x3to disks and 8.1tb of space, so 3 disks are used and 1 for redundancy right ? But what is this redundancy disk ? It has only partial datas in it ? It can't save 8tb on 3tb so what happened if one of the 3 disks died ?

The data is spread amongst the disk. RAID doesn't provide backup, it provides redundancy (except for non-redundant RAID i.e. RAID-0).

Think of X-RAID2 with four equally sized disks like this:
Disks A, B, C and D
A = 1/3 B + 1/3 C + 1/3 D
B = 1/3 A + 1/3 C + 1/3 D
C = 1/3 A + 1/3 B + 1/3 D
D = 1/3 A + 1/3 B + 1/3 C

cyse wrote:
I understand that i need to backup my data on a removable hdd, that's what i planned to do but i choosed raid1 in first instancez because i thought if one disk die i just need to buy another one and replace in the bay. So why xraid2 is better if i can loose my datas easier ?

There is a trade-off more storage space, but any one disk can fail and all data remains intact using X-RAID2 (with 2 or more disks). You have to choose the RAID type that suits you best.

cyse wrote:
Also, which disk is the redundant one now in x-raid2 ? which number is the redundant disk ? If i remove it when nas is on, what happened ? Because i want to use it to backup my files before reset factory the nas.

Parity is spread amongst the disks. Would recommend sticking with X-RAID2. If you remove a disk your array will be non-redundant and an additional disk failure will render your volume dead. To recover data from your volume outside of the NAS you would need X-1 disks (where X is the number of disks in the NAS when the volume is redundant). Also hot-swap is for replacing dead disks or disks you no longer need to use. Anyway a single disk from a 3 or more disk X-RAID2 array is useless by itself for trying to recover data as you need X-1 disks.

Backup to a USB disk or some place else.