Have you tried to re-add the drive?

mdadm --add /dev/sdc1 /dev/md0

Please recheck the syntax I'm on my phone. 

Thanks. 

-------- Original message --------
From: Mark Mitchell <mark.russel.mitchell at gmail.com> 
Date:06/01/2014  18:15  (GMT-06:00) 
To: tclug-list <tclug-list at mn-linux.org> 
Subject: [tclug-list] This Software RAID newbie's array is missing a drive. 

I built my first RAID array about a week ago, kinda vaguely
understanding what I'm doing.  4 3 TB drives in a RAID 5.  It's been
working fine, and I've been slowly filling it up.

Took the machine apart today to upgrade case cooling and power supply.
When I brought it back up, I got this in the boot messages;

md/raid:md0: raid level 5 active with 3 out of 4 devices, algorithm 2

My 1st thought was that I didn't get everything hooked up (I had to
disconnect most of the drives to get the power connections on the
board), several reboots later, I'd confirmed that the system was
seeing all the drives;

root at debian:/var/log# lsblk
NAME        MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda           8:0    0 931.5G  0 disk
├─sda1        8:1    0    14G  0 part  /
├─sda2        8:2    0  37.3G  0 part  [SWAP]
└─sda3        8:3    0 880.3G  0 part  /home
sdb           8:16   0   2.7T  0 disk
└─sdb1        8:17   0   2.7T  0 part
  └─md0       9:0    0   8.2T  0 raid5
    └─md0p1 259:0    0   8.2T  0 md    /srv/media
sdc           8:32   0   2.7T  0 disk
└─sdc1        8:33   0   2.7T  0 part
sdd           8:48   0   2.7T  0 disk
└─sdd1        8:49   0   2.7T  0 part
  └─md0       9:0    0   8.2T  0 raid5
    └─md0p1 259:0    0   8.2T  0 md    /srv/media
sde           8:64   0   2.7T  0 disk
└─sde1        8:65   0   2.7T  0 part
  └─md0       9:0    0   8.2T  0 raid5
    └─md0p1 259:0    0   8.2T  0 md    /srv/media
sr0          11:0    1   7.3G  0 rom

So, it looks like the partition on sdc isn't being seen as a raid
partition anymore.

FWIW;
root at debian:/var/log# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdb1[0] sde1[4] sdd1[2]
      8790405120 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [U_UU]
      bitmap: 4/22 pages [16KB], 65536KB chunk

unused devices: <none>

So, what's my next step?  Looks to me like I need to recreate sdc1 as
a raid partition, then regrow the array.  Is this correct?

In the meantime, I'm reading what I can and copying data off the
array.  Nothing critical, but it'd be annoying to lose.

Let me know if I left out required information.

Thanks.
Mark
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