The scenario: my RAID crashed because I’ve messed around with the partition table of one of the disks in there. This results in a RAID array not being able to assemble itself because the superblock of the messed up device is invalid. The trick is pretty easy: just recreate the whole RAID with mdadm. The existing metadata will not be overwritten, the current information is just replicated. I used to have a simple RAID1, but I’ve now recreated it as an incomplete RAID5 (--level=5, --raid-devices=2) as the missing disk is soon to be bought.

$ mdadm --create /dev/md0 --level=5 --raid-devices=2 /dev/<original> /dev/<crashed>

If you like to stick with a RAID1, and not doing the migration to RAID5 along the way, just use --level=1 instead. I’m not really sure if the order of the disks matter and I’m not brave enough to find it out.

Tomorrow I’m going to buy the next disk for the RAID to make sure the redundancy level is alright. Generally I’m pretty amazed that this kind of setup is so robust. Even me messing around with it can’t bring it down.