What logic did you use to determine which disk to pull? Your append seems to indicate they were selected at random (try one, then another - that's not good as you mess up the last access times and further degrade the array...)

To really know which disk is which, just before powering down, simplest is to run "smartctl -a /dev/sdx | grep Serial" - where sdx is the disk in question. then check the disks for that serial number... alternatively "hdparm -I /dev/sdx | grep 'Serial\ Number' " will also provide the same information.

Note that swapping disks may change the way linux enumerates the disk logical numbers, and understand that the disk number and "RaidDevice" numbers in the output from mdadm bear no direct relationship with physical slots or bays.

Do we assume from your error that the system is no longer bootable? If so, you will need to boot a live linux system on CD, DVD, SD card or USB drive to find out the state of your raid array - impossible to know from the information rovided whether it can be salvaged...

Raid has saved data here several times - but you need to understand totally what you are doing, not panic, and ask questions first before touching anything if not 100% sure...

You removed /sdc, why do you say it didn't help? You need to partition as raid, and then insert with raid utility
By removing the other drive you broke your raid array and now it will not automatically reassemble. It is possible to reassemble the array with a missing drive and use the force switch with the raid utilities.
I've learned to save my raid configuration to a file with all the serial numbers, and yes that was the hard way!

Thanks for the info.

[root@server ~]# smartctl -a /dev/sds | grep Serial

-bash: smartctl: command not found

[root@server ~]# hdparm -I /dev/sda | grep 'Serial\ Number'

-bash: hdparm: command not found

How do I get around this?

Do you need "smartmontools"?

Nick Howitt wrote:

Do you need "smartmontools"?

I don't know. How do I find out? If I do need it: how do I get it?

[root@server ~]# yum whatprovides hdparm

Loaded plugins: clearcenter-marketplace, fastestmirror

ClearCenter Marketplace: fetching repositories...

Loading mirror speeds from cached hostfile

 * clearos: mirror2.timburgess.net

 * clearos-addons: mirror2.timburgess.net

 * clearos-centos: download2.clearsdn.com

 * clearos-centos-updates: download2.clearsdn.com

 * clearos-extras: mirror2.timburgess.net

 * clearos-fast-updates: download2.clearsdn.com

 * clearos-updates: mirror2.timburgess.net

 * contribs: download2.clearsdn.com

 * private-clearcenter-dyndns: download3.clearsdn.com:80

 * private-clearcenter-plex: download4.clearsdn.com:80

 * private-clearcenter-roundcubemail: download2.clearsdn.com:80

 * private-clearcenter-smart-monitor: download2.clearsdn.com:80

 * private-clearcenter-static-vpn: download1.clearsdn.com:80

hdparm-9.43-4.el6.x86_64 : A utility for displaying and/or setting hard disk

                         : parameters

Repo        : clearos

Matched from:







hdparm-9.43-4.el6.x86_64 : A utility for displaying and/or setting hard disk

                         : parameters

Repo        : clearos

Matched from:







hdparm-9.43-4.el6.x86_64 : A utility for displaying and/or setting hard disk

                         : parameters

Repo        : clearos-centos

Matched from:







hdparm-9.43-4.el6.x86_64 : A utility for displaying and/or setting hard disk

                         : parameters

Repo        : installed

Matched from:

Other       : Provides-match: hdparm

[root@server ~]# yum whatprovides */smartctl

Loaded plugins: clearcenter-marketplace, fastestmirror

ClearCenter Marketplace: fetching repositories...

Loading mirror speeds from cached hostfile

 * clearos: mirror2.timburgess.net

 * clearos-addons: mirror2.timburgess.net

 * clearos-centos: download2.clearsdn.com

 * clearos-centos-updates: download2.clearsdn.com

 * clearos-extras: mirror2.timburgess.net

 * clearos-fast-updates: download2.clearsdn.com

 * clearos-updates: mirror2.timburgess.net

 * contribs: download2.clearsdn.com

 * private-clearcenter-dyndns: download4.clearsdn.com:80

 * private-clearcenter-plex: download4.clearsdn.com:80

 * private-clearcenter-roundcubemail: download2.clearsdn.com:80

 * private-clearcenter-smart-monitor: download2.clearsdn.com:80

 * private-clearcenter-static-vpn: download1.clearsdn.com:80

clearos/filelists_db                                     | 1.0 MB     00:00

clearos-addons/filelists_db                              |  72 kB     00:00

clearos-centos/filelists_db                              | 6.3 MB     00:19

clearos-centos-updates/filelists_db                      | 5.3 MB     00:14

clearos-extras/filelists_db                              | 122 kB     00:00

clearos-fast-updates/filelists_db                        | 1.3 kB     00:00

clearos-updates/filelists_db                             | 409 kB     00:00

1:smartmontools-5.43-1.el6.x86_64 : Tools for monitoring SMART capable hard

                                  : disks

Repo        : clearos-centos

Matched from:

Filename    : /usr/sbin/smartctl







1:smartmontools-5.43-1.el6.x86_64 : Tools for monitoring SMART capable hard

                                  : disks

Repo        : clearos-extras

Matched from:

Filename    : /usr/sbin/smartctl







1:smartmontools-5.43-1.el6.x86_64 : Tools for monitoring SMART capable hard

                                  : disks

Repo        : installed

Matched from:

Filename    : /usr/sbin/smartctl

So:

yum install hdparm smartmontools

I did:

smartctl -a /dev/sda | grep Serial

smartctl -a /dev/sdb | grep Serial

smartctl -a /dev/sdc | grep Serial

smartctl -a /dev/sdd | grep Serial

And it runs out 1 was a, 2 was b, 3 was c and 4 was d.

Thanks.