Today I noticed that there are bunch of messages complaining about the RAID array (it's a software RAID10), so I started looking into it but need help because I'm unsure if I interpret the status output correctly (I've kinda forgotten the actual RAID set-up because the machine is at a remote location and I configured it about a year or two ago)... if I remember correctly the system was suppose to have 8x 2TB disks, but that's about all I can remember.System mail: N 14 root@edmedia.loca  Wed May 25 21:30   32/1059  Fail event on /dev/md/0:EDMedia N 15 root@edmedia.loca  Thu May 26 06:25   30/1025  DegradedArray event on /dev/md/0:EDMedia N 16 root@edmedia.loca  Thu May 26 06:25   30/1025  SparesMissing event on /dev/md/0:EDMediaThe bit that's specifically confusing me, now that I'm looking at the outputs, is this:Number   Major   Minor   RaidDevice State   0       0        0        0      removedDoes it mean that a disk has been removed (or that it dropped from the array)? Should I try re-adding '/dev/sda1' to it? And is there any way I can tell that '/dev/sda1' was part of '/dev/md0' without adding a partitioned disk in-use by something, only to make things worse?Status outputs:'mdadm -D /dev/md0' output:/dev/md0:        Version : 1.2  Creation Time : Mon Feb  8 23:15:33 2016     Raid Level : raid10     Array Size : 2197509120 (2095.71 GiB 2250.25 GB)  Used Dev Size : 1465006080 (1397.14 GiB 1500.17 GB)   Raid Devices : 3  Total Devices : 2    Persistence : Superblock is persistent  Intent Bitmap : Internal    Update Time : Thu Sep  1 19:54:05 2016          State : clean, degraded Active Devices : 2Working Devices : 2 Failed Devices : 0  Spare Devices : 0         Layout : near=2     Chunk Size : 512K           Name : EDMEDIA:0           UUID : 6ebf98c8:d52a13f0:7ab1bffb:4dbe22b6         Events : 4963861    Number   Major   Minor   RaidDevice State       0       0        0        0      removed       1       8       17        1      active sync   /dev/sdb1       2       8       33        2      active sync   /dev/sdc1'lsblk' output:NAME                       MAJ:MIN RM   SIZE RO TYPE   MOUNTPOINTsda                          8:0    0   1.4T  0 disksda1                       8:1    0   1.4T  0 partsdb                          8:16   0   1.4T  0 disksdb1                       8:17   0   1.4T  0 part  md0                      9:0    0     2T  0 raid10    md0p1                259:0    0   1.5M  0 md    md0p2                259:1    0 244.5M  0 md     /boot    md0p3                259:2    0     2T  0 md      EDMedia--vg-root   253:0    0     2T  0 lvm    /      EDMedia--vg-swap_1 253:1    0    16G  0 lvm    [SWAP]sdc                          8:32   0   1.4T  0 disksdc1                       8:33   0   1.4T  0 part  md0                      9:0    0     2T  0 raid10    md0p1                259:0    0   1.5M  0 md    md0p2                259:1    0 244.5M  0 md     /boot    md0p3                259:2    0     2T  0 md      EDMedia--vg-root   253:0    0     2T  0 lvm    /      EDMedia--vg-swap_1 253:1    0    16G  0 lvm    [SWAP]sdd                          8:48   0   1.4T  0 disksdd1                       8:49   0   1.4T  0 partsdj                          8:144  0 298.1G  0 disksdj1                       8:145  0 298.1G  0 partsr0                         11:0    1  1024M  0 rom'df' output:Filesystem      1K-blocks       Used Available Use% Mounted on/dev/dm-0      2146148144 1235118212 801988884  61% /udev                10240          0     10240   0% /devtmpfs             1637644      17124   1620520   2% /runtmpfs             4094104          0   4094104   0% /dev/shmtmpfs                5120          0      5120   0% /run/locktmpfs             4094104          0   4094104   0% /sys/fs/cgroup/dev/md0p2         242446      34463    195465  15% /boot'watch -n1 cat /proc/mdstat' output:Every 1.0s: cat /proc/mdstat                                                                                                                                                           Thu Sep  1 21:26:22 2016Personalities : [raid10]md0 : active raid10 sdb1[1] sdc1[2]      2197509120 blocks super 1.2 512K chunks 2 near-copies [3/2] [_UU]      bitmap: 16/17 pages [64KB], 65536KB chunkunused devices: <none>
Raid array 'clean, degraded'?
raid;array
Inspected the system logs as rudimeier suggested and found out there had been a power outage event back at May, after which the RAID array errors started popping up. Since this is a software RAID10 (1+0), I'm thankful only the spare disk flew out of the array instead of the whole array irreversibly crashing. After doing a couple HDD tests with the trusty old Hiren's boot CD and just for variety - Partition Wizard bootable... all suspicious disks checked out with no errors/issues.I erased (with Partition Wizard bootable, so that the disk would be unformatted and unpartitioned) and then re-added the spare using:mdadm --add /dev/md0 /dev/sda1