I have a QNAP TS-421 that has a failed RAID5. The Web interface shows the RAID as in degraded mode. Drive 2 has failed. I’ve replaced drive 2 (/dev/sdb) with a new 2TB drive to match the other (working) three drives.
The Web interface fails to rebuild the array. Whilst the data is still accessible on the RAID it is in degraded mode and has no fault tolerance.
Resorting to SSH / PUTTY to fix this:
Check each of the drives:
with:
fdisk -l /dev/sda fdisk -l /dev/sdb fdisk -l /dev/sdc fdisk -l /dev/sdd
Each drive yeilds similar to:
[~] # fdisk -l /dev/sda Disk /dev/sda: 2000.3 GB, 2000398934016 bytes 255 heads, 63 sectors/track, 243201 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Device Boot Start End Blocks Id System /dev/sda1 1 66 530125 83 Linux /dev/sda2 67 132 530142 83 Linux /dev/sda3 133 243138 1951945693 83 Linux /dev/sda4 243139 243200 498012 83 Linux
Check the status of the RAID with mdadm
mdadm --detail /dev/md0 /dev/md0: Version : 01.00.03 Creation Time : Tue Jan 24 17:34:17 2017 Raid Level : raid5 Array Size : 5855836608 (5584.56 GiB 5996.38 GB) Used Dev Size : 1951945536 (1861.52 GiB 1998.79 GB) Raid Devices : 4 Total Devices : 3 Preferred Minor : 0 Persistence : Superblock is persistent Update Time : Wed Jun 7 09:25:11 2017 State : clean, degraded Active Devices : 3 Working Devices : 3 Failed Devices : 0 Spare Devices : 0 Layout : left-symmetric Chunk Size : 64K Name : 0 UUID : f15b5712:01afc7a3:61851d59:5d6f8393 Events : 4391 Number Major Minor RaidDevice State 0 8 3 0 active sync /dev/sda3 1 0 0 1 removed 2 8 35 2 active sync /dev/sdc3 3 8 51 3 active sync /dev/sdd3
As expected we can see /dev/sdb3 is missing from above.
Make sure that the drive we want to add into the array isn’t mounted using
umount /dev/sdb3
Next add the drive into the array (this will delete any existing data on the drive being added /dev/sdb in this case)
[~] # mdadm /dev/md0 --add /dev/sdb3
Now check the details of the array again
[~] # mdadm --detail /dev/md0
/dev/md0:
Version : 01.00.03
Creation Time : Tue Jan 24 17:34:17 2017
Raid Level : raid5
Array Size : 5855836608 (5584.56 GiB 5996.38 GB)
Used Dev Size : 1951945536 (1861.52 GiB 1998.79 GB)
Raid Devices : 4
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Wed Jun 7 09:27:20 2017
State : clean, degraded, recovering
Active Devices : 3
Working Devices : 4
Failed Devices : 0
Spare Devices : 1
Layout : left-symmetric
Chunk Size : 64K
Rebuild Status : 0% complete
Name : 0
UUID : f15b5712:01afc7a3:61851d59:5d6f8393
Events : 4421
Number Major Minor RaidDevice State
0 8 3 0 active sync /dev/sda3
4 8 19 1 spare rebuilding /dev/sdb3
2 8 35 2 active sync /dev/sdc3
3 8 51 3 active sync /dev/sdd3
Here we can see that /dev/sdb3 is now part of the array and rebuilding
The Web GUI should now also show the array rebuilding.
The unit has TWO drives configured as a RAID 1 mirror (this will only work with units with two or less drives fitted).
here: 
