On Thu, 30 Apr 2015 21:25:04 +0200 Peter van Es [off-list ref] wrote:

Neil,

thanks. I followed your instructions (slightly modified as my version of mdadm did not support the --data-offset stanza). /dev/sdd was the 3rd drive and I had physically removed the 4th drive from my server.

I managed to restart the array. Then I replaced the failing drive, created partitions the same as on /dev/sda and added it to the two arrays.

It is now rebuilding for the data array, and will be done in 440 minutes.... It appears that I've lost nothing important...

Excellent.

One question: I did spot that the Array UUID has changed on the Create command. Is there any way of getting it back to the old value ?

Why would you want to?

But I think you can.  Firstly stop the array (so you need to be booted from a
USB or similar) and then

 mdadm --assemble /dev/mdWHATEVER --update=uuid --uuid=your:favo:rite:nums ..list.of.devices..

NeilBrown

Peter

quoted

Before doing this, double check that the names have changed, so check that
 mdadm --examine /dev/sda2
shows

quoted

    Array UUID : 1f28f7bb:7b3ecd41:ca0fa5d1:ccd008df
  Device Role : Active device 0

(among other info) and  that 
 mdadm --exmaine /dev/sdb2
show the same Array UUID and

quoted

  Device Role : Active device 1

Then run

mdadm -C /dev/md1 -l5 -n4 --data-offset=262144s --metadata=1.2 --assume-clean \
 /dev/sda2 /dev/sdb2 missing /dev/sde2