Hello,
about 4 years ago I set up the NAS with 6TB disks, which I configured in RAID5.
Since a few days I've been getting a warning message from the daily checks done by OMV, so I investigated and noticed that RAID 5 is on 5 disks (instead of all 6 installed ones); also, the missing disk has a "Warning" with "Bad sectors".
I guess I need to replace the disk, but what would be the correct procedure?
Could you point me to a guide?
Below you can see - from OMV GUI - the RAID configuration (labeled as "degraded."

Here you can see the detail of the missing disk, namely "sdb":

Finally, here is some data taken from CLI to help understand the overall situation:
root@diynas-omv:/srv/dev-disk-by-label-RAID5/# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [raid0] [raid1] [raid10]
md0 : active raid5 sdc[4] sdd[0] sdf[1] sde[5] sda[2]
29301969920 blocks super 1.2 level 5, 512k chunk, algorithm 2 [6/5] [UUU_UU]
bitmap: 40/44 pages [160KB], 65536KB chunk
unused devices: <none>
root@diynas-omv:/srv/dev-disk-by-label-RAID5/# blkid
/dev/sdf: UUID="add77410-4af9-84b4-4a1b-f4c9a53296ae" UUID_SUB="540cd822-1281-aead-7f6f-ca4f0842dc80" LABEL="DIYNAS-OMV:RAID5" TYPE="linux_raid_member"
/dev/nvme0n1p5: UUID="21dbc86c-cfc1-4b4a-a9ed-566388d70285" TYPE="swap" PARTUUID="5b2a273a-05"
/dev/nvme0n1p1: UUID="975fe369-9535-4459-80f8-f86795bdcb26" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="5b2a273a-01"
/dev/sdd: UUID="add77410-4af9-84b4-4a1b-f4c9a53296ae" UUID_SUB="9e5d7091-039a-77b5-77b8-c3450cc7cac8" LABEL="DIYNAS-OMV:RAID5" TYPE="linux_raid_member"
/dev/sdb: UUID="add77410-4af9-84b4-4a1b-f4c9a53296ae" UUID_SUB="f6d205c8-bc1a-5cd1-6a8a-7458a4b0c817" LABEL="DIYNAS-OMV:RAID5" TYPE="linux_raid_member"
/dev/md0: LABEL="RAID5" UUID="b143f942-1f22-42e0-b92d-4e4c557ff36c" BLOCK_SIZE="4096" TYPE="ext4"
/dev/sde: UUID="add77410-4af9-84b4-4a1b-f4c9a53296ae" UUID_SUB="f5ddc7d4-d721-6796-e75e-733251246271" LABEL="DIYNAS-OMV:RAID5" TYPE="linux_raid_member"
/dev/sdc: UUID="add77410-4af9-84b4-4a1b-f4c9a53296ae" UUID_SUB="1e112d20-45bd-32d0-9ebf-69bea2a73f9b" LABEL="DIYNAS-OMV:RAID5" TYPE="linux_raid_member"
/dev/sda: UUID="add77410-4af9-84b4-4a1b-f4c9a53296ae" UUID_SUB="93903863-bbf5-8435-c86f-c1dc338c5bb7" LABEL="DIYNAS-OMV:RAID5" TYPE="linux_raid_member"
root@diynas-omv:/srv/dev-disk-by-label-RAID5/# fdisk -l | grep "Disk "
Disk /dev/nvme0n1: 232.89 GiB, 250059350016 bytes, 488397168 sectors
Disk model: Samsung SSD 970 EVO Plus 250GB
Disk identifier: 0x5b2a273a
Disk /dev/sdc: 5.46 TiB, 6001175126016 bytes, 11721045168 sectors
Disk model: ST6000NM0095
Disk /dev/sdf: 5.46 TiB, 6001175126016 bytes, 11721045168 sectors
Disk model: ST6000NM0095
Disk /dev/sdd: 5.46 TiB, 6001175126016 bytes, 11721045168 sectors
Disk model: ST6000NM0095
Disk /dev/sda: 5.46 TiB, 6001175126016 bytes, 11721045168 sectors
Disk model: ST6000NM0115-1YZ
Disk /dev/sde: 5.46 TiB, 6001175126016 bytes, 11721045168 sectors
Disk model: ST6000NM0115-1YZ
Disk /dev/sdb: 5.46 TiB, 6001175126016 bytes, 11721045168 sectors
Disk model: ST6000NM0115-1YZ
Disk /dev/md0: 27.29 TiB, 30005217198080 bytes, 58603939840 sectors
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root@diynas-omv:/srv/dev-disk-by-label-RAID5/# cat /etc/mdadm/mdadm.conf
# This file is auto-generated by openmediavault (https://www.openmediavault.org)
# WARNING: Do not edit this file, your changes will get lost.
# 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>
# instruct the monitoring daemon where to send mail alerts
MAILADDR my-mail@my-domain.com
MAILFROM root
# definitions of existing MD arrays
ARRAY /dev/md0 metadata=1.2 name=DIYNAS-OMV:RAID5 UUID=add77410:4af984b4:4a1bf4c9:a53296ae
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root@diynas-omv:/srv/dev-disk-by-label-RAID5/# mdadm --detail --scan --verbose
ARRAY /dev/md0 level=raid5 num-devices=6 metadata=1.2 name=DIYNAS-OMV:RAID5 UUID=add77410:4af984b4:4a1bf4c9:a53296ae
devices=/dev/sda,/dev/sdc,/dev/sdd,/dev/sde,/dev/sdf
Thank you very much!