On Fri, Oct 07, 2016 at 05:37:32PM +0200, Santiago DIEZ wrote:

First thing I did is ddrescue the remaining partitions sd[abc]10 .
ddrescue did not stumble into any read error so I assume all remaining
partitions are perfectly safe.

So ... don't you still have a good copy?

You only killed one of them, right? Did not make same mistake twice?

There comes my mistake: I ran the --create command instead of --assemble :

================================================================================
# mdadm --create --verbose /dev/md1 --raid-devices=4 --level=raid5
--run --readonly /dev/loop0 /dev/loop1 /dev/loop2 missing

mdadm: layout defaults to left-symmetric
mdadm: layout defaults to left-symmetric
mdadm: chunk size defaults to 512K
mdadm: /dev/loop0 appears to contain an ext2fs file system
       size=5778741888K  mtime=Sat Sep  3 11:00:22 2016
mdadm: /dev/loop0 appears to be part of a raid array:
       level=raid5 devices=4 ctime=Wed Jan 25 09:08:11 2012
mdadm: /dev/loop1 appears to be part of a raid array:
       level=raid5 devices=4 ctime=Wed Jan 25 09:08:11 2012
mdadm: /dev/loop2 appears to be part of a raid array:
       level=raid5 devices=4 ctime=Wed Jan 25 09:08:11 2012
mdadm: size set to 1926115840K
mdadm: automatically enabling write-intent bitmap on large array
mdadm: creation continuing despite oddities due to --run
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md1 started.
================================================================================

You had 0.90 metadata before, that is metadata at the end of the device. 
1.2 metadata is at the start of the device (4K from start). So you have 
overwritten your filesystem superblock...

You can --create again with --metadata=0.90 --chunk=64K options and 
see what is left to the rescue.

But it would be much better if you still had your good copy of the original.

Regards
Andreas Klauer