So I changed my case and I don't know why but one my SATA port does not work anymore. So only 5 of my 6 RAID5 disks were recognized. The volume was automounted anyway and I already wrote new files on it. So I shut-downed the computer and just used the other free SATA port, but unfortunately mdadm didn't picked it as my 6th disk.
It just counts my disk as removed even though the system recognized it as device sdd. I saw a readd command in the man page of mdadm but I don't know if it's appropriate for my case. I don't want to make a stupid mistake. How can I restore the volume to 6 active disks?
Additional information about my raid5 volume :
root@Debian8:~# mdadm --detail /dev/md127 Persistence : Superblock is persistent Update Time : Mon May 23 21:24:10 2016 State : clean, degraded Name : openmediavault:RAID5en6x6 UUID : 016ab622:4c52176d:33c85eea:9226dd9e Events : 13571 Number Major Minor RaidDevice State 0 8 0 0 active sync /dev/sda 2 0 0 2 removed 2 8 16 2 active sync /dev/sdb 3 8 64 3 active sync /dev/sde 4 8 32 4 active sync /dev/sdc 5 8 80 5 active sync /dev/sdf