Hi Joseba,


On 11.10.17 12:25, Joseba Ibarra wrote:

The md0 is ext4 formated. But now, I can even start the SO when all 
the disk are pluged. One of them is corrupt. It makes an odd sound at 
the starting. However if I unplug that disk the system start fine, 
however no RAID is detected and after assemble it says:


root@grafico:/home/jose# mdadm --detail /dev/md0
/dev/md0:
                 Version : 1.2
      Creation Time : Sat Aug 5 23:10:50 2017
            Raid Level : raid5
     Used Dev Size : 976629760 (931.39 GiB 1000.07 GB)
        Raid Devices : 4
        Total Devices : 3
          Persistence : Superblock is persistent

         Update Time : Thu Sep 21 13:34:35 2017
                     State : active, degraded, Not Started
      Active Devices : 3
   Working Devices : 3
      Failed Devices : 0
      Spare Devices : 0

                  Layout : left-symmetric
          Chunk Size : 512K

                   Name : servidor:0
                    UUID : 0b44a3b8:83eafabc:644afc87:bdb5b1f3
                  Events : 3109

    Number   Major   Minor   RaidDevice State
-       0        0        0      removed
1       8       17        1      active sync   /dev/sdb1
2       8       33        2      active sync   /dev/sdc1
3       8       49        3      active sync   /dev/sdd1


I'm not sure how to continue, since i don't see the RAID. GParted see 
the disks, however doesn't see the md0 and I'm bit scared if I lost 
the data content.

Let me see if i understand you correctly

- with all 4 disks plugged in, your system does not boot
- with the broken disk unplugged, it boots (and from your description it 
is really broken, no DISK recovery possible unless by specialised company)
- raid does not get assembled during boot, you do a manual assembly?
      -> please provide the command you are using

from the log above, you should be able to do a mount of /dev/md0 which 
would auto-start the raid.

If that works, the next step would be to check the health of the other 
disks. smartctl would be your friend.
Another useful action would be to copy all important data to a backup 
before you add a new disk to replace the failed disk.

Cheers

Rudy