Originally Posted by tkrin
The full output of `mdadm -D /dev/md2` can be see below. I've never run into this situation before.
Anyone have any pointers on how I can reduced the Raid Devices from 4 to 3? I have tried
mdadm /dev/md2 -r failed
mdadm /dev/md2 -r detached
You might want to try:
mdadm /dev/md2 --fail detached --remove detached
As per the man page in mdadm: Any devices which are components of /dev/md2 will be marked as faulty and then remove from the array.
It seems that at some point there was a 4th device added to this array, I've never reduced an array, as per mdadm man page you can do it using --grow and --array-size:
When decreasing the number of devices, the size of the array will also decrease. If there was data in the array, it could get destroyed and this is not reversible. To help prevent accidents, mdadm requires that the size of the array be decreased first with mdadm --grow --array-size. This is a reversible change which simply makes the end of the array inaccessible. The integrity of any data can then be checked before the non-reversible reduction in the number of devices is request.
Can you check your /etc/mdadm.conf file to see if it shows the extra device in the configuration.
Before looking into those options I would have a full backup just in case.