State of the raid: clean,degraded

Hello, my sistem OMV is notifiying RAID "clean,degraded".

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root@omv:~# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sda[0] sdc[2] sdb[1]
11720661504 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UUU_]
bitmap: 29/30 pages [116KB], 65536KB chunk

unused devices: <none>

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root@omv:~# blkid
/dev/sdb: UUID="cfcdc47b-839f-3ada-5522-ced3ab9c26d8" UUID_SUB="ec958142-e8ca-e9c9-3336-81f2c12f9cf7" LABEL="Panoramix:Raid5" TYPE="linux_raid_member"
/dev/sda: UUID="cfcdc47b-839f-3ada-5522-ced3ab9c26d8" UUID_SUB="f465aad1-a6ee-2b13-86c7-768de12cfb27" LABEL="Panoramix:Raid5" TYPE="linux_raid_member"
/dev/sdc: UUID="cfcdc47b-839f-3ada-5522-ced3ab9c26d8" UUID_SUB="170488df-051e-0b35-c4e1-ec79f8a06abd" LABEL="Panoramix:Raid5" TYPE="linux_raid_member"
/dev/sdd1: LABEL="WDUsb" UUID="582af5dc-d61c-458f-aa32-d40cb4664497" TYPE="ext4" PARTUUID="414b3331-6b68-4204-82e2-6782ac80b63a"
/dev/sde1: UUID="cf4eb673-11d8-4de8-a17a-a9745e6c57e1" TYPE="ext4" PARTUUID="690bbffe-01"
/dev/sde5: UUID="2ab62fd2-7e0a-4455-ae7f-fbb69ad9fcdc" TYPE="swap" PARTUUID="690bbffe-05"
/dev/md0: LABEL="WDRed" UUID="c2888fb6-c83c-4ea3-adcf-863d9f4c262a" TYPE="xfs"

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root@omv:~# fdisk -l | grep "Disk "
Disk /dev/sdb: 3,7 TiB, 4000787030016 bytes, 7814037168 sectors
Disk /dev/sda: 3,7 TiB, 4000787030016 bytes, 7814037168 sectors
Disk /dev/sdc: 3,7 TiB, 4000787030016 bytes, 7814037168 sectors
Disk /dev/sdd: 3,7 TiB, 4000787030016 bytes, 7814037168 sectors
Disk identifier: A9B8A8C6-8956-4E3A-B43E-4E8818B2B5C2
Disk /dev/sde: 29,7 GiB, 31914983424 bytes, 62333952 sectors
Disk identifier: 0x690bbffe
Disk /dev/md0: 10,9 TiB, 12001957380096 bytes, 23441323008 sectors
Disk /dev/sdf: 3,7 TiB, 4000752599040 bytes, 7813969920 sectors

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root@omv:~# cat /etc/mdadm/mdadm.conf
# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

# by default, scan all partitions (/proc/partitions) for MD superblocks.
# alternatively, specify devices to scan, using wildcards if desired.
# Note, if no DEVICE line is present, then "DEVICE partitions" is assumed.
# To avoid the auto-assembly of RAID devices a pattern that CAN'T match is
# used if no RAID devices are configured.
DEVICE partitions

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# definitions of existing MD arrays
ARRAY /dev/md0 metadata=1.2 name=omv:Raid5 UUID=cfcdc47b:839f3ada:5522ced3:ab9c26d8

# instruct the monitoring daemon where to send mail alerts

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root@omv:~# mdadm --detail --scan --verbose
ARRAY /dev/md0 level=raid5 num-devices=4 metadata=1.2 name=omv:Raid5 UUID=cfcdc47b:839f3ada:5522ced3:ab9c26d8
devices=/dev/sda,/dev/sdb,/dev/sdc

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The OMV System is mount in HP GEN8 with 4 hdd WD RED of 4TB in RAID5 (sda,sdb,sdc,sdd). The OS OMV is mount in microsd 32gb (sde). And finally a hdd usb of 4tb for backup (sdf).

Thanks!!!

In case it helps the info of RAID5 en OMV is (in red mark the error):Version : 1.2
Creation Time : Fri Oct 27 21:05:16 2017
Raid Level : raid5
Array Size : 11720661504 (11177.69 GiB 12001.96 GB)
Used Dev Size : 3906887168 (3725.90 GiB 4000.65 GB)
Raid Devices : 4
Total Devices : 3
Persistence : Superblock is persistent

Intent Bitmap : Internal

Update Time : Sat Dec 2 14:08:29 2017
State : clean, degraded
Active Devices : 3
Working Devices : 3
Failed Devices : 0
Spare Devices : 0

Layout : left-symmetric
Chunk Size : 512K

Name : Panoramix:Raid5 (local to host Panoramix)
UUID : cfcdc47b:839f3ada:5522ced3:ab9c26d8
Events : 34530

Number Major Minor RaidDevice State
0 8 0 0 active sync /dev/sda
1 8 16 1 active sync /dev/sdb
2 8 32 2 active sync /dev/sdc
6 0 0 6 removed